Rad Onc ACR Questions Radiation/Biology

5 Gy of X-rays is delivered to mammalian cells in vitro using a megavoltage linear accelerator. What is the MOST likely observation 6 hours after irradiation? (A) Approximately 30 γ H2AX subnuclear foci are counted per cell per Gy (B) The BRCA1 protein localizes to a subset of the total number of radiation-induced subnuclear foci (C) Using the comet assay, the length of the "tail" is reduced in the irradiated cells compared to unirradiated cells (D) Using pulsed-field gel electrophoresis, the fraction of DNA released from the agarose plug is at its maximum relative to other time points

A .The majority of DNA double strand breaks are repaired via non-homologous end-joining, with a minority repaired via homologous recombination. BRCA1, which is involved in homologous recombination repair, localizes to a subset of radiation induced DNA repair foci. Most of the DNA damage capable of being repaired already has been by 6 hours after irradiation. In fact, 40-50% of double strand breaks are repaired within 30 minutes and may not produce radiation-induced foci at all. An average of 20-40 double-strand breaks per Gy is produced initially, however far fewer ioniallyfoc will persist 6 hours post-radiation. Using pulsed-field gel electrophoresis, the maximum DNA released from the agarose plug would occur immediately after radiation before repair occurs, not 6 hours later. The comet assay measures fragmented DNA released from the nucleus when an electric field is applied. A longer comet "tail" reflects more DNA double strand breaks than a shorter tail.

For the treatment of prostate carcinoma with low-dose rate brachytherapy, the American Brachytherapy Society recommends slightly different prescription doses depending on whether I-125 sources or Pd-103 sources are used. The reason for a slightly higher recommended prescription dose for I-125 sources is the fact that I-125 has a: (A) longer half-life. (B) smaller seed size. (C) lower anisotropy constant. (D) larger average photon energy.

A A longer half-life Dose is delivered over a longer period of time for a source with a longer half-life, necessitating a higher total dose.

Which of the following is a characteristic of homologous recombination repair (HRR)? HRR: (A) occurs predominantly in the S and G2 phases of the cell cycle. (B) is an error-prone process. (C) does not require physical contact with an undamaged chromosome or chromatid. (D) signaling pathways include ATM as an effector protein

A HRR occurs *predominantly in the S and G2 phases of the cell cycle*. HRR is a *high-fidelity process* that requires *physical contact* with an undamaged chromosome or chromatid. *ATM is a sensor protein in the HRR signaling pathway*.

Which of the following is a feature of radiation-induced necrosis? (A) Generates a local inflammatory response (B) Is an ATP-independent process (C) Is associated with sequence-specific DNA cleavage (D) Can be confused with the early phase of apoptosis

A Over the last decade, it has been shown that necrosis is an ATP-dependent process. *Pro-necrotic signaling through membrane-bound death receptors, kinases (RIP1, MAPK family members), and DNA repair proteins (PARP1) all deplete cellular ATP levels*, and the balance between necrotic and apoptotic cell death pathways may be dependent on cellular ATP levels.

Which of the following systems is the MOST secure option for backup of data from a treatment planning system? (A) A second hard drive on the treatment planning server (B) A redundant archival facility that is located offsite (C) An external hard drive stored in the Physicist's office (D) A USB flash drive that is kept next to the treatment planning station for quick access.

B The most secure way to archive data is through the use of a dedicated off-site facility.

Clinical photodynamic therapy (PDT) shares which characteristic with clinical radiotherapy? (A) Generation of immediate DNA double-strand breaks (B) Generation of reactive oxygen species (C) Requirement for drug sensitization (D) Ability to reach deep-seated tumors

B Of the options listed above, *PDT shares only the ability to generate reactive oxygen species (in photodynamic drug-sensitized cells)* with clinical radiotherapy. PDT does not directly induce DNA DSBs. require drug photosensitization, (unlike RT which does not). Has restricted body penetration due to the physics of light absorption in human tissue (unlike RT ).

To generate an x-ray survival curve for a new cell line, 4 Petri dishes were seeded with 102, 103, 104 and 105 cells, and irradiated with 0, 3, 6 and 9 Gy, respectively. At the end of a 2 week incubation period, a total of 40 colonies was counted on each dish. This cell line's survival curve: (A) has a D0 of 3 Gy. (B) is exponential. (C) has a broad shoulder. (D) has an n of 15.

B The finding that each subsequent dose produces one log's difference in cell survival suggests that the survival curve for this cell line is exponential. Exponential survival curves are characterized by having no shoulders and extrapolation numbers of 1.0. The D0 for this cell line is 1.3 Gy. (D10 = 3 Gy = 2.3 * D0).

Which of the following cysteine-aspartic proteases is an effector caspase? (A) Caspase 2 (B) Caspase 3 (C) Caspase 8 (D) Caspase 9

B. Caspases 2, 8, 9 and 10 are initiator caspases that cleave inactive pro-forms of effector caspases to activate them. The effector caspases (caspases 3, 6, 7), once activated, cleave other protein substrates within the cell that trigger apoptosis. Caspases 4 and 5 are not currently classified as either initiator or effector.

Linear energy transfer (LET) is defined as the: (A) total amount of energy deposited per unit track length. (B) average amount of energy deposited per unit track length. (C) average number of tracks needed to deposit a unit of energy. (D) specific energy of a charged particle depositing energy over a unit track length.

B. The track average LET is calculated by dividing the particle's total track into equal length increments, and determining the average energy deposition in each increment. Please note that this is an average amount of energy deposited per unit track length; in reality, particular track length increments do not necessarily all receive exactly the same energy deposition. Specific energy refers to the energy of an incident charged particle, and not to the amount of energy deposited

For radiation with an LET of 100 keV/μm, the average separation between ionization events is closest to the diameter of which of the following cellular components? (A) Nucleus (B) Ribosome (C) DNA helix (D) Mitochondria

C 100 keV/μm is the LET that yields the highest RBE for the production of biological effects. This is thought to be because the average separation between ionizing events approximates the diameter of the DNA double helix (~2 nm).

For patients younger than 10 years of age who received a cumulative dose of 500-600 mSv from head CT scanning, about how many scans are projected to cause one excess case of leukemia and one excess brain tumor per year 10 years after the initial scan? (A) 100 (B) 1,000 (C) 10,000 (D) 100,000

C A recent epidemiological study by Pearce et al. examined cancer incidence, mortality, and loss to follow-up in a large cohort of UK pediatric patients examined with head CT from 1985 to 2008. A positive association between radiation dose and excess relative risk for leukemia and brain tumors was noted. Use of CT scans in children to deliver cumulative doses of about 50 mGy might almost triple the risk of leukaemia and doses of about 60 mGy might triple the risk of brain cancer. Because these cancers are relatively rare, the cumulative absolute risks are small: in the 10 years after the first scan for patients younger than 10 years, one excess case of leukaemia and one excess case of brain tumour per 10 000 head CT scans is estimated to occur. Nevertheless, although clinical benefits should outweigh the small absolute risks, radiation doses from CT scans ought to be kept as low as possible and alternative procedures, which do not involve ionizing radiation, should be considered if appropriate

Which statement about apoptotic cell death is true? (A) It is a cell-type independent process. (B) Bax is an oncogenic protein that suppresses apoptosis. (C) DNA double stranded breaks produced during apoptosis cause the characteristic ladders seen on electrophoretic gels. (D) During the apoptosis process, cells flatten and spread out in order to maintain physicalcontact with neighboring cells.

C -Apoptosis is cell-type dependent; different types of cells are more or less prone to undergo apoptosis in response to an injury or stressor. -Bcl-2 is an oncogenic protein that suppresses apoptosis, whereas Bax is a tumor suppressor protein that facilitates the apoptosis process. -Cells undergoing apoptosis typically round up and detach from their neighbors

There are several normal tissues that, in order to carry out their functions or in response to certain disease states, display one or more malignant phenotypes, yet are not considered cancer. Which of the following pairs of normal tissue type and malignant phenotype is correct? (A) bone marrow : angiogenic (B) placenta : limitless replicative potential (C) endometriosis : metastatic (D) psoriasis : apoptosis resistant

C Endometriosis is a consequence of the abnormal growth of endometrial cells outside of the uterus. Akin to a metastatic process, these cells spread around the pelvis and seed structures called "endometriosis implants" that can continue to grow and spread further. Implants are most commonly found on the Fallopian tubes, outer surfaces of the uterus or intestines, and on the surface lining of the pelvic cavity. Normal Bone marrow is not angiogenic per se, and only shows an increase in vascular density secondary to hematopoietic malignancies or myelodysplastic syndromes. Placental cells can be invasive, but are not characterized by replicative immortality. Psoriasis is a disorder of keratinocyte hyperproliferation in the epidermis, but not apoptosis-resistance.

Which of the following statements about autophagy is TRUE? (A) Transgenic mice bearing bi-allelic deletions of autophagy-related genes are cancer prone. (B) It has no influence on radiation or chemosensitivity. (C) It plays a tumor-suppressive role in normal cells by safeguarding them against metabolic stress. (D) Mutations in the autophagy-associated gene BECN1 are noted in 5-10% in breast cancers.

C. In normal cells, autophagy serves as a tumor suppressor by protecting cells from metabolic stress that might otherwise lead to genomic instability. This is accomplished through the controlled degradation of mitochondria and aggregated proteins. Bi- allelic deletion of autophagy-related genes is embryonic lethal in transgenic mice, although mono-allelic loss does lead to cancer proneness. Mutations in one autophagy-associated gene, BECN1, are found in 40-75% of human breast, ovarian and prostate tumors, and are associated with aggressive tumors and poor prognosis. Genetic knockdown of autophagy-related proteins increases the effectiveness of radiation or chemotherapy in most cases, although there are exceptions.

What accounts for the higher surviving fraction when delaying the subculture of quiescent cells in vitro after a large, single dose of X-rays compared to cells cultured immediately after irradiation? (A) Base excision repair (B) Sublethal damage recovery (C) Nucleotide excision repair (D) Potentially lethal damage recovery

D. Operationally, potentially lethal damage recovery (PLDR) is defined as the increase in cell survival after a single dose of low LET radiation noted when plating for colony formation is delayed for several hours - leaving cells in a quiescent state in the interim - compared to cells plated immediately after irradiation. PLDR is one manifestation of the repair of DNA double strand breaks; the extent to which this recovery occurs depends on the post-irradiation environmental conditions, that is, conditions which favor PLDR are those that discourage cell proliferation (e.g., due to quiescence, presence of inhibitory drugs, etc.), and vice versa. In a simplistic sense, this can be viewed as a "competition" between continued cell proliferation and DNA repair, processes that normally don't occur simultaneously.

Which of the following protein(s) is involved in ionizing radiation-induced autophagy? (A) Bcl-2 (B) Caspases (C) Cytochrome c (D) Protein kinases

D. Protein kinases are involved in mediating both ionizing radiation-induced apoptotic and autophagic cell death. Bcl-2, cytochrome c, and caspases are only associated with apoptotic cell death.

Japanese survivors of the atomic bombings who were irradiated in utero what time period is Microcephaly most significant?

Microcephaly. A significant effect of radiation on the frequency of small heads is observed only in the periods 0 to 7 and 8 to 15 weeks postovu- lation. No significant excess was seen among persons exposed at 16 weeks or more. The pro- portion of exposed persons with microcephaly increases with dose, and there is little evidence for a threshold in dose

Which endpoint associated with radiation-induced -H2AX foci is thought to be of the greatest clinical utility for radiotherapy? (A) Baseline number of y-H2AX foci prior to irradiation (B) Number of y-H2AX foci 1 hour after irradiation (C) Residual numbers of y-H2AX foci 24 hours after irradiation (D) Co-localization of y-H2AX foci with other DNA double strand break markers

Which endpoint associated with radiation-induced -H2AX foci is thought to be of the greatest clinical utility for radiotherapy? (A) Baseline number of y-H2AX foci prior to irradiation (B) Number of y-H2AX foci 1 hour after irradiation (C) Residual numbers of y-H2AX foci 24 hours after irradiation (D) Co-localization of y-H2AX foci with other DNA double strand break markers

When is the fetal period in humans and what is the effect of radiation on the human embryo?

fetal period, extends from about 6 weeks onward in the human. irradiation during the fetal stages, including effects on the hema- topoietic system, liver, and kidney, all occurring, however, after relatively high radiation doses. The effects on the developing gonads have been documented particularly well. There appears at present to be little correspondence between the cellular and functional damage as a function of dose, but doses of a few tenths of a gray as a minimum are necessary to produce fertility changes

From which study population was the genetic doubling dose estimate for humans derived? (A) Irradiated mice (B) Irradiated Drosophila (C) Radiation therapy patients (D) Japanese atomic bomb survivors

A Genetic doubling dose estimates were derived from mouse studies, with the data corrected as much as possible for differences between mice and humans. The present-day consensus is that these estimates were too low, however human data on radiation's hereditary risk are not sufficiently robust to provide a better estimate of genetic doubling dose. ICRP reduced the dose limit for radiation workers to, effectively, what it is today— a maximum of 50 mSv per year. This was based entirely on heritable effects in the fruit fly, Drosophila.

Withers' skin colony assay requires that a "moat" of necrosis surround an "island" of keratinocytes to be irradiated because the: (A) moat prevents the migration of cells from outside the radiation field into the island. (B) dying cells in the moat provide a feeder layer for the surviving cells in the island. (C) dying cells in the island are absorbed into the moat, leaving more room for survivors to repopulate. (D) cytokines released by the dying cells in the moat stimulate repopulation of the surviving cells in the island.

A In order for the skin colony assay to properly assess clonogenic survival of irradiated epithelial cells, it is necessary to minimize artifacts that could interfere with the measurement of inherent radiosensitivity. Migration and proliferation of clonogenic cells from outside the radiation field would artificially increase the surviving fraction of cells in the irradiated island. To prevent this from happening, a moat of necrosis is used to isolate the island.

Platinum chemotherapeutics function by: (A) crosslinking DNA strands. (B) intercalating between DNA bases. (C) disrupting microtubule function. (D) inhibiting thymidylate synthase.

A Platinum chemotherapeutics (including cisplatin, carboplatin and oxaliplatin) are among the most commonly used classes of cancer drugs clinically. The platinum compounds function by *crossing linking DNA through covalent bonds between Pt complexes and the DNA bases*, with preference for guanine residues.

Which of the following provides the best operational definition of PLD repair? (A) An increase in survival following the delayed plating post-irradiation of density-inhibited cells relative to immediate plating. (B) An increase in survival following the delayed delivery of a second dose to density-inhibited cells relative to their receiving a second dose immediately after the first. (C) A decrease in survival following the delayed plating post-irradiation of density-inhibited cells relative to their immediate plating. (D) A decrease in survival following the addition of oxygen to hypoxic cells within 3 μsec post-irradiation relative to hypoxic cells that receive no oxygen.

A Potentially lethal damage (PLD) is an operational definition that describes changes in survival with altered post-irradiation conditions. *If survival is increased PLD is said to be repaired* while if there is a decrease in survival PLD is said to be fixed as "fixed in place". The classic example of PLDR repair occurs when *survival increases after the delayed post-irradiation subculture of plateau phase cells.* The delayed application of a second dose of radiation (split dose experiment) is an example of sublethal damage repair. The *addition of oxygen to hypoxic cells very shortly after irradiation is likely to decrease survival.*

Which normal tissue has the HIGHEST tolerance to re-irradiation? (A) Skin (B) Kidney (C) Bladder (D) Spinal cord

A Retreatment tolerance is governed by the total dose of radiation delivered as the initial treatment, the initial treatment volume, the ability of the tissue at risk of injury to recover, and the time interval elapsed since the initial round of radiotherapy. Of the tissues listed above, radiation-induced skin damage has been studied the most, and shows restoration of almost full radiation tolerance, provided at least a couple of months has passed since the first treatment course. Spinal cord and lung are also able to recover partial retreatment tolerance, particularly for lower initial radiation doses, and provided more than 6 months has elapsed since the initial treatment. Kidney and bladder have significant difficulty recovering from late functional damage, even when the initial total dose delivered is not to full tolerance.

Which of the following is needed to calculate the tumor cell cycle time in vivo? (A) Percent labeled mitoses curve (B) Growth fraction (C) Cell loss factor (D) Potential doubling time

A The percent labeled mitoses technique is used to determine the overall cell cycle times and the durations of the specific cell cycle phases for amenable normal tissues and tumors in vivo. Options B, C, and D are all cell cycle kinetic parameters used to study tumor growth kinetics.

Which of the following is an example of a parallel organ? (A) Lung (B) Esophagus (C) Spinal cord (D) Optic chiasm

A To model the response of OARs to radiation, tissues are assumed to be made up of functional subunits (FSUs). The response of these FSUs is dependent on whether the tissue is organized in a serial, parallel, or serial-parallel manner. For parallel tissues (e.g. lung), complication rates are proportional to the volume of tissue exposed to doses in excess to the tissue tolerance. In contrast, for serial tissues (e.g. spinal cord, esophagus, and optic chiasm), it is assumed that irradiation of even a small volume of tissue to a dose above tolerance, could be detrimental. Of the options presented, the only parallel organ listed is the lung.

Compared to normal vasculature, tumor vasculature is: (A) porous and leaky. (B) less tortuous. (C) spaced at regular intervals. (D) resistant to ionizing radiation

A Tumor vasculature has many unique characteristics when compared to normal vasculature, including, among others, large pores and poorly organized, leaky structures. It is also more tortuous and spaced irregularly through the tumor.

Which provides a direct measure of tumor oxygenation? (A) Interstitial oximetry (B) Pimonidazole immunohistochemistry (C) Carbonic anhydrase IX (CA-IX) immunohistochemistry (D) Epstein-Barr virus-encoded RNA (EBER) immunohistochemistry

A While both CA-IX and pimonidazole are used as markers of hypoxia within tissues, only interstitial oximetry can directly measure oxygen levels within a tissue. EBER staining is a marker used for the diagnosis of Epstein Barr Virus-associated cancers. Studying tumor hypoxia remains important as tumors containing a viable, radiobiologically hypoxic fraction are expected to be 2-3 fold more resistant to radiation than fully oxygenated tumors. However, measuring tumor hypoxia has not yet found its way into routine clinical practice.

Wortmannin functions as a potent radiosensitizer by: (A) inhibiting DNA double-strand break repair. (B) inhibiting p53 function. (C) activating the cellular necrosis pathway. (D) synchronizing cells in radiosensitive phases of the cell cycle.

A Wortmannin is an inhibitor of PI3K and PI3K-related kinases. It is known to inhibit DNA-PK, which in turn impairs the repair of DNA double stranded breaks by the process of non-homologous end-joining.

The whole-organ radiation tolerance dose for the kidney is low, whereas small volumes can tolerate much higher doses. This is because the kidney: (A) contains functional subunits that are arranged in parallel. (B) shows a strong dependence on overall treatment time. (C) manifests radiation injury within 3 months of irradiation. (D) has a small functional reserve

A By convention, normal organs are classified as "serial" or "parallel", by analogy with electrical circuits. A serial organ is one where a severe injury *at any anatomical point* in the structure will produce a severe functional loss. The classical example of this is *the spinal cord.* If there is major damage to the cord at any level, from a small dosimetric hot spot for example, all function can be lost below that level. *Examples of parallel organs are the kidneys and the liver*.

How would a tumor with a short cell cycle time, a low growth fraction and a high cell loss rate behave before and in response to radiation therapy. (A) Grow slowly; regress rapidly (B) Grow slowly; regress slowly (C) Grow rapidly; regress slowly (D) Grow rapidly; regress rapidly

A. A tumor with a low growth fraction and high cell loss rate would grow slowly overall, despite containing a subpopulation of cells with short cell cycle times and that are proliferating rapidly. On the assumption that the radiation therapy would be preferentially toxic to the rapidly growing cells, and possibly, impair the ability of the survivors to repopulate rapidly, this would "unmask" the high rate of cell loss, causing the tumor to regress rapidly during and after treatment.

Which of the following DNA damage assays is used to detect the presence of single strand breaks? (A) Alkaline elution (B) Comet assay (C) Neutral elution (D) Pulsed-field gel electrophoresis

A. DNA filter elution assays are useful in studies of DNA strand break production and rejoining caused by exposure to a wide variety of toxins and carcinogens, including ionizing radiation. The method involves forcing DNA fragments through small-pored membranes; the more fragmented the DNA (i.e., contains more strand breaks), the quicker it will pass through the membrane. The assay can be performed using a neutral or alkaline pH buffer solution, with the latter optimized to detect DNA single strand breaks.

Based on Casarett's classification system for tissue radiosensitivity, which of the following classes of cells is the MOST radioresistant? (A) Fixed postmitotic (B) Reverting postmitotic (C) Vegetative intermitotic (D) Differentiating intermitotic

A. Group I: Vegetative Intermitotic Divides constantly with no differentiation. Includes basal epithelial cells (skin, intestinal crypts, etc), undifferentiated hematopoietic stem cells and germ cells. Vegetative intermitotic cells divide regularly, but do not undergo differentiation (i.e., stem cell-like). These are the most radiosensitive cell types. Group II: Differentiating Intermitotic Divides for a finite amount of cycles before differentiating into a non-dividing cell. Includes all of the cells that are intermediate between stem cells and differentiated cells. For example, myelocytes, spermatogonia, etc. Differentiating intermitotic cells still proliferate, but with increasing differentiation between successive cell divisions, and are somewhat less radiosensitive. Group III: Reverting Postmitotic A normally non-dividing cell that retains the potential to divide ("revert"). Includes liver, kidneys and glandular tissues such as pancreas, adrenal, thyroid and pituitary. Reverting postmitotic cells are differentiated (at least to some extent) and normally do not divide, but can be recruited back into the active cell cycle in response to a tissue injury. Once the injury is resolved, these cells revert back to being differentiated and stop dividing, making them more radioresistant. Group IV: Fixed Postmitotic A permanently non-dividing cell. Includes permanent cells such as nerves and muscles, as well as short-lived differentiated cells such as neutrophils, red blood cells, and superficial epithelial cells. Fixed postmitotic cells are terminally differentiated and therefore have lost the ability to divide. And because radiation-induced cell killing is, in most cases, linked to cell division, these cells tend to be the most radioresistant.

Which protein kinase does NOT phosphorylate histone H2AX at sites of DNA double strand breaks? (A) mTOR (B) ATM (C) DNA-PK (D) ATR

A. Of the four PI3K-like serine/threonine protein kinases mentioned, only mTOR has not been shown to phosphorylate histone H2AX at serine 139

Prenatal death is MOST likely to occur if the embryo or fetus is exposed to ionizing radiation during what period after conception? (A) 0-2 weeks (B) 3-12 weeks (C) 13-23 weeks (D) 24-36 weeks

A. Prenatal death is most likely to occur if the embryo is exposed to ionizing radiation within the first 8 days after conception. Per Hall 1. Lethal effects are induced by radiation before or immediately after implantation of the embryo into the uterine wall or are induced after increasingly higher doses during all stages of intrauterine development, to be expressed either before birth (prenatal death) or at about the time of birth (neonatal death). 2. Malformations are characteristic of the period of major organogenesis in which the main body structures are formed, and especially of the most active phase of cell multiplication in the relevant structures. 3. Growth disturbances and growth retardation, without malformations are induced at all stages of development but particularly in the latter part of pregnancy. The principal factors of importance are the dose and the stage of gestation at which it is delivered. Dose rate is also of significance because many pathologic effects on the embryo are reduced significantly by reducing the dose rate. Gestation is divided into 3 phases: preimplantation- 0 through 9 day organogenesis- 10 days through 6 weeks fetal period. > 6 weeks 1. Large doses of radiation (2.5 Gy) delivered to the human embryo before 2 to 3 weeks of gestation are not likely to produce severe abnormalities in most children born, although a considerable number of the embryos may be resorbed or aborted. 2. Irradiation between 4 and 11 weeks of ges- tation would lead to severe abnormalities of many organs in most children. 3. Irradiation between 11 and 16 weeks of gestation may produce a few eye, skeletal, and genital organ abnormalities; stunted growth, microcephaly, and mental retardation are fre- quently present. Preimplantation: Preimplantation is the most sensitive stage to the lethal effects of radiation. Radiation has an "all-or-nothing" effect of radiation because if the number of cells in the conceptus is small and their nature is not yet specialized. The International Commission on Radiological Protection (ICRP) Publication 103 confirms embryonic susceptibility to the lethal ef- fects of irradiation in the preimplantation period of embryonic development, but suggests that at doses less than 100 milligrays (mGy), such lethal effects will be very infrequent in the human. Organogenesis During organogenesis, the principal effect of radiation in small rodents is the production of various congenital anomalies of a structural nature. A dose of about 2 Gy of x-rays to the mouse embryo during the period of maxi- mum sensitivity can result in a 100% incidence of malformations at birth.a dose of about 2 Gy of x-rays to the mouse embryo during the period of maxi- mum sensitivity can result in a 100% incidence of malformations at birth Fetal Period The remainder of pregnancy, the fetal period, about 6 weeks onward in the human. Much higher doses of radiation are required to cause lethality during this period than at earlier stages of development, although the ir- radiated early fetus exhibits the largest degree of permanent growth retardation, in contrast to the embryo in early organogenesis, which exhibits the most temporary growth retardation, which is evident at term but from which the animal is able to recover later

Which of the following is an advantage of proton radiotherapy compared to photon therapy? (A) Lower integral dose (B) Minimal range uncertainty in soft tissue (C) Increased tumor cell kill (D) Higher RBE of ~1.5

A. Proton beam radiation generally achieves satisfactory target coverage with 2-3 beams, and therefore does not produce a low dose bath as seen with standard photon therapy which requires more fields. Protons at clinically used energies have a low LET and a RBE of only 1.0-1.1.

The key difference in the molecular pathways of radiation-induced apoptosis and mitotic catastrophe is that: (A) apoptosis relies on p53 signaling pathways while mitotic catastrophe does not. (B) mitotic catastrophe is triggered by DNA damage while apoptosis is not. (C) apoptosis causes caspase activation while mitotic catastrophe does not. (D) a defective G2 checkpoint frequently accompanies apoptosis, but not mitotic catastroph

A. The key difference between the two radiation induced cell death pathways is that apoptosis signals through p53 while mitotic catastrophe is p53 independent. Both can lead to caspase activation and both are cellular responses to DNA damage. A defective G2 checkpoint frequently accompanies mitotic catastrophe, but not apoptosis.

Which of the following human genetic diseases is characterized by an increased level of sister chromatid exchanges? (A) Bloom syndrome (B) Cockayne syndrome (C) Xeroderma pigmentosum (D) Nijmegen breakage syndrome

A. The mutated gene responsible for Bloom Syndrome is a RecQ helicase that is involved in homologous recombination repair, specifically by repairing damage at stalled replication forks. This defect leads to exchanges with undamaged templates in either homologous chromosomes or sister chromatids. The genes involved in Xeroderma Pigmentosum and Cockayne Syndrome assist with excision repair while that of Nijmegen Breakage Syndrome works with non-homologous end joining. Cockayne syndrome (CS), also called Neill-Dingwall syndrome, is a rare and fatal autosomal recessive neurodegenerative disorder characterized by growth failure, impaired development of the nervous system, abnormal sensitivity to sunlight (photosensitivity), eye disorders and premature aging.

Clinical outcomes for radiotherapy patients with persistently hypoxic tumors are typically worse because hypoxia: (A) selects for tumor cells with more aggressive phenotypes. (B) stimulates compensatory tumor cell repopulation. (C) prevents the formation of toxic reactive oxygen species. (D) activates pro-apoptotic signaling pathways in tumor cells.

A. Tumor hypoxia develops secondary to abnormalities in tumor vasculature and the tendency for tumors to outgrow their own blood supply. Hypoxia does activate pro-apoptotic signaling pathways in normal cells, however tumor cells, due to genomic instability, typically have lost these pathways. Hypoxia per se does not stimulate tumor cell proliferation, although reoxygenation of previously hypoxic cells can. What hypoxia (in particular, cycles of hypoxia and reoxygenation) does do however is cause genomic instability, which in turn tends to select for tumor cells that are already resistant to apoptotic cell death, are more invasive and more metastatic.

As the dose per fraction is reduced and number of fractions increased, the RBE for neutrons: (A) remains unchanged. (B) increases. (C) increases but then plateaus. (D) decreases.

B *As the dose per fraction is reduced and number of fractions increased, the RBE for neutrons increases*. This occurs because the amount of sublethal damage recovery, and therefore the size of the shoulder on the cell survival, is less for high LET radiations than for X-rays. *This difference is magnified the more highly fractionated (or the lower the dose rate) the treatment is*

The dose response curve shape for exchange-type chromosome aberrations following acute doses of low LET radiation is linear-quadratic. Lowering the dose rate will: (A) have no effect on the linear (αD) or quadratic (βD2) components. (B) have no effect on the linear (αD) component, but decrease the quadratic (βD2) component. (C) decrease the linear (αD) component, but have no effect on the quadratic (βD2) component. (D) decrease both the linear (αD) and quadratic (βD2) components.

B A chromosome exchange results when a cells misrepairs two or more chromosome breaks that are spatially and temporally proximate by joining the break ends to inappropriate partners. Two biophysical processes can produce these conditions, the first where both breaks are produced by the same energy absorption event *(single track, linear kinetics, "α")* and the second where each break is produced by *a separate and independent electron track (multi-track, curvilinear kinetics, "β")*. *Reducing the dose rate spaces out in time the formation of these tracks allowing the cell to restitute a chromosome break before a second spatially proximate break can be formed.* By lowering the dose rate the production of single track exchanges (α) is unaffected while the production of multi-track exchanges (β) is curtailed.

Which of the following proteins is an apical kinase in the cellular response to ionizing radiation? (A) Hypoxia-inducible factor 1 (HIF-1) (B) Ataxia telangiectasia mutated (ATM) (C) p53 (D) Retinoblastoma (Rb)

B Ataxia telangiectasia mutated (ATM) is a key apical kinase that *orchestrates the cellular DNA damage response* to ionizing radiation, *including the initial sensing* of DNA damage, *mobilization* of DNA repair proteins, and the *stopping of cell cycle progression* while repair is occurring.

Base transversion mutations, identified by next-generation sequencing in cells from a small cell lung carcinoma, are caused by which of the following? (A) Radon (B) Tobacco (C) Mutant EGFR (D) Asbestos

B Base transversion mutations, such as G>T or C>A, are associated with tobacco smoke carcinogens, a risk factor for the development of small cell lung carcinoma. Several recent articles have highlighted the power of next-generation sequencing methods to analyze the genomic profile of cancer cells.

Which characteristic makes cancer stem cells an important determinant of resistance to RT? (A) Absence from metastatic sites (B) Ability to repopulate during treatment (C) Presence of multiple DNA repair deficiencies (D) Can initiate carcinogenesis following a stem cell transplant

B Cancer stem cells are pluripotent cells with a high replicative potential, that give rise both to more stem cells and to tumor progenitor cells that become heterogeneous genotypically and phenotypically. Notonlyaretheythoughttoberesistanttoconventionaltherapies,butalsotheyhave the ability to repopulate an entire tumor with progenitors. Cancer stem cells are implicated in the metastatic process and may be enriched in sites of metastatic disease, however they have no role in cancers that may arise following a stem cell transplant.

Which action increases the steepness of the tumor control probability curve for a patient population included in a dose escalation clinical trial? (A) Inclusion of patients with very small tumors (B) Removal of patients with uncommon radiosensitive tumors (C) Inclusion of patients with very large tumors (D) Removal of patients with the most common tumor stage

B Clinically defined groups of tumors are usually characterized by *shallow dose-response curves, resulting from heterogeneity among individual dose-response curves*. This means that the *more heterogeneous a study population is, the flatter the dose-response curve becomes* and the more difficult it is to detect an improvement in treatment outcome as a result of an increase in the radiation dose. Factors that cause a flattening of the curve are not limited to: -different radiosensitivities of the tumors -differences in the initial number of clonogens (ie, tumor size), -differing hypoxic fractions, -inhomogeneities of dose distribution.

For 80 keV/μm carbon ions, the dose response curve for dicentric chromosome aberrations is: (A) linear-quadratic. (B) linear and steep. (C) linear and shallow. (D) initially linear, then threshold.

B Dose response curves for two-hit chromosome aberrations like dicentrics are typically linear-quadratic in shape for low LET radiations like x-rays and linear for high LET radiations like carbon ions. And because high LET carbon ions are more biologically effective per unit dose than low LET x-rays, it follows that the dose response curve for the induction of dicentrics would not only be linear, but also steep.

36. A fetus that receives 25 cGy of X-rays after the 16th week of gestation is at elevated risk for: (A) miscarriage. (B) carcinogenesis. (C) growth retardation. (D) congenital malformations.

B From about 16 weeks gestation until birth, non-cancer health effects in the fetus are very unlikely, especially for doses below ~50 cGy. However, doses to a fetus in the range of 5-50 cGy are associated with an increased risk of childhood cancer (leukemia in particular).

How does the oxygen enhancement ratio (OER) change with changing LET? (A) Increase in OER with increase in LET (B) Decrease in OER with increase in LET (C) No change in OER with increase in LET (D) No change in OER with decrease in LET

B OER, the oxygen enhancement ratio, equals the ratio of radiation doses with and without oxygen to yield the same biological endpoint. *Since high LET radiation causes more direct, irreparable DNA damage, it is less reliant on the oxygen "fixing" additional free radicals*. Therefore, the OER decreases with increasing LET. References: Chapter 6 in Hall and Giaccia, Radiobiology for the Radiologist, 7th Edition, 2012.

Which of the following effects of radiation is stochastic? (A) Hematopoietic syndrome (B) Cancer (C) Cataracts (D) Skin erythema

B Of the options listed above, only cancer is a stochastic effect, while the others are deterministic effects. The latter show dose thresholds and can vary in severity, unlike stochastic effects. Stochastic effects. Effects that occur by chance and which may occur without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. In the context of radiation protection, the main stochastic effect is cancer. Deterministic effect. Health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Deterministic effects generally result from the receipt of a relatively high dose over a short time period.

Which of the following is an advantage of using thermoluminescent dosimeters (TLDs) for in-vivo measurements? (A) Their response is independent of energy. (B) Their small size makes them convenient for use. (C) They create a permanent record of the dose delivered. (D) One dosimeter provides a 2D representation of the dose distribution.

B The *small size of TLDs makes then very convenient to position accurately* and they *do not alter the dose distribution in any significant manner*. They are energy dependent, with the* response being much larger in the kV range* than in the MV range. *Once read, they lose most of their information and therefore do not create a permanent record of dose*. Finally, one TLD also give a single reading, not a 2D distribution like film would.

The estimated genetic doubling dose for humans is based on mouse data. However recent human data from Japanese a-bomb survivors suggest that these mouse-derived estimates may be: (A) too high. (B) too low. (C) approximately correct. (D) meaningless.

B The genetic doubling dose is the dose of radiation that produces an induced mutation frequency equal to the spontaneous frequency, hence doubling the overall mutation rate. These estimates have been based on mouse studies historically, and while comparisons with the available human data require judicious use of correction factors to take into account various differences between the systems, the general consensus is that the *mouse estimates are too low.*

The The Oxygen Enhancement Ratio (OER) is HIGHEST for cells irradiated with: (A) neutrons. (B) electrons. (C) carbon ions. (D) alpha particles.

B The more severe, less repairable DNA damage caused by exposure to high LET radiation does not depend on oxygen free radicals to further augment the injury, as is the case for low LET radiation. Therefore, the The Oxygen Enhancement Ratio (OER) is high for low LET radiations like electrons or X-rays, and lower (or equal to 1.0) for high LET radiations like carbon ions, neutrons or alpha particles.

For TBI in preparation for a bone marrow transplant, radiation is delivered at a low dose rate in order to: (A) allow time for hypoxic stem cell niches to reoxygenate fully. (B) reduce the incidence of early and late effects in normal tissues. (C) take advantage of DNA repair deficiencies common to many tumor cells. (D) increase the sensitivity of hematopoietic stem cells by cell cycle redistribution.

B When radiation is used for bone marrow ablation prior to a transplant, the radiation is delivered at a low dose rate (and/or fractionated) in order to reduce the incidence of early and late effects in normal tissues. In the early days of bone marrow transplantation for example, radiation-induced lung fibrosis was a frequent and sometimes fatal complication observed when the treatment was at the more standard, higher dose rates used for radiotherapy. In addition, some patients would begin to manifest symptoms associated with the whole body irradiation syndromes, including fairly prompt nausea and vomiting that could interfere with subsequent treatment.

Which statement about the use of heat shock protein 90 (Hsp90) small molecule inhibitors as radiation sensitizers is TRUE? (A) Different Hsp90 inhibitors are uniform in their biological effects. (B) More than 40 Phase I-III clinical trials with Hsp90 inhibitors have been completed to date, and others are ongoing. (C) Loss of Hsp90 chaperone function would not cause degradation of large, multi-unit proteins or multi-protein complexes. (D) A cocktail of different Hsp90 inhibitors would be required to effectively target multiple oncogenic kinases and transcription factors.

B. More than 40 Phase I-III clinical trials have already been completed using Hsp90 inhibitors in combination with radiation and/or chemotherapy. Additional clinical trials are ongoing. Heat shock protein 90 (Hsp90) is a molecular chaperone upregulated in response to cellular stress, and is among the most abundant proteins expressed in cells. The functions of Hsp90 include assisting in protein folding, transport, stabilization and degradation (through the ubiquitin-proteasome pathway) in response to stress (e.g., heat shock, exposure to DNA damaging agents, etc.). Hsp90 stabilizes a number of oncoproteins involved in tumor survival and growth, including many signal transduction kinases and some transcription factors. Thus, inhibition of Hsp90 could lead to the simultaneous depletion of a multiple oncoproteins, which makes the development of Hsp90 inhibitors particularly attractive. All of the clinical Hsp90 inhibitors studied to date are specific in their target, i.e. they bind exclusively to Hsp90 and two related heat shock proteins. However, Hsp90 inhibitors are markedly pleiotropic, causing degradation of over 200 client proteins and impacting critical multiprotein complexes

The mechanism by which cells undergo radiation-induced senescence involves cells: (A) dying during or soon after an aberrant mitosis. (B) permanently arresting in one phase of the cell cycle. (C) dying spontaneously within several hours of irradiation. (D) becoming genomically unstable and prone to neoplastic transformation.

B. Senescence is a condition of permanent cell cycle arrest that cannot be reversed by the addition of growth factors. It can occur naturally (as part of the normal aging process for example) as well as be induced by genotoxic or oncogenic stress. The remaining answer options are characteristics of apoptosis (A) and mitotic catastrophe (B). Senescence is one way a cell can avoid becoming genomically unstable and prone to transformation.

Which of the following proteins is a well-characterized therapeutic target based on its ability to promote tumor angiogenesis? (A) VEGF receptor 1 (B) VEGF receptor 2 (C) EGF receptor 1 (D) EGF receptor 2

B. The VEGF receptor 2 mediates many of the known cellular responses to VEGF, most notably, angiogenesis. The VEGF receptor 1 protein is produced by a wide variety of cell types, but is less well studied. It is thought to be involved with modulating VEGF receptor 2 signaling and can also act as a "decoy receptor" by sequestering VEGF from VEGF receptor 2 binding. The EGF receptors 1 and 2 are known to modulate other cellular phenotypes, including migration, adhesion and proliferation

Which of the following is NOT a characteristic of irradiated ataxia telangiectasia cells? (A) Contain more residual DNA damage than wild-type cells (B) Have steep, shoulderless X-ray survival curves (C) Lack the extrinsic apoptotic pathway (D) Are deficient in G1 checkpoint control

C ATM is tumor suppressor, "caretaker" gene whose biallelic inactivation or loss causes the *extreme sensitivity* to ionizing radiation and cancer proneness characteristic of patients with this clinical syndrome. The *ATM protein is an apical kinase* that participates in the *sensing of DNA damage, the recruitment of repair proteins, and the coordination of the processes of DNA repair and cell cycle checkpoint control*. Cells from patients with *AT do not experience the G1 phase checkpoint* and exhibit *high levels of residual DNA damage after irradiation and extreme radiosensitivity* as a consequence of having *steep, shoulderless x-ray survival curves.* The apoptotic response to DNA damage is often defective in AT, *the extrinsic apoptotic pathway (which is unrelated to the DNA damage response and p53 activation) remains intact.*

In comparing the Chernobyl and Fukushima nuclear power plant accidents, which statement is TRUE? (A) Faulty design contributed to the Chernobyl accident, but not the Fukushima accident. (B) There were approximately 30 acute radiation deaths at Chernobyl, and 10 at Fukushima. (C) The psychosocial and economic impacts of both accidents are the worst of the "radiation effects". (D) The maximum dose rate detected at or near the damaged reactors was higher in Fukushima than Chernobyl.

C Arguably, the socioeconomic and psychological impacts of the Chernobyl accident have long since exceeded the impact of the several thousand childhood thyroid cancer cases that were induced. To date, this is also true of the Fukushima accident, although it is still too early to tell whether excess cases of cancer will develop. However, most experts feel that radiation- induced cancer cases will be far fewer in Fukushima, not only because the amount of radiation released in the accident was much smaller, but also because, especially with respect to thyroid cancer in children, possibly contaminated milk was immediately embargoed, unlike in Chernobyl.

Which of the following best describes the change in relative biological effectiveness (RBE) as a function of LET? The RBE at: (A) 10 keV/μm 100 keV/μm (B) 10 keV/μm ≈ 50 keV/μm (C) 50 keV/μm 1000 keV/μm (D) 50 keV/μm

C As LET increases, ionizations are spatially grouped closer together. *This spacing is most effective at producing biological damage at an LET of about 100 KeV/μm*. *Above 100 KeV/μm* this level, more dose is deposited in a sensitive target than is required leading to dose "wastage". Another way to think about this is that the *number of ionizations per unit dose is constant. Increasing the LET increases the number of ionizations along a particle track while reducing the number of tracks required to produce a specific dose*. Targets hit by these tracks will certainly be damaged but as there are fewer tracks, the number of targets hit will be reduced.

Which of the following statements is true about the dose rate effect for X-rays? (A) The dose rate effect is a manifestation of the repair of potentially lethal damage. (B) The biological effectiveness of 30 Gy delivered as 5 daily doses of 5 Gy is less than for 30 Gy delivered as a single dose. (C) Survival curves for low dose rate or highly fractionated irradiation are roughly exponential. (D) For repair-competent cells, as the dose rate increases, survival curves become progressively shallower.

C As dose *rates drop, survival curves for low LET types of radiation become shallower* and increasingly exponential. *The dose rate effect is a consequence of the repair of sublethal damage, not potentially lethal damage.* Typically, biological effectiveness decreases as the dose rate decreases (or amount of fractionation increases). *For repair competent cells, survival curves become shallower as the dose rate decreases, not increases.*

The in vivo-in vitro assay involves the irradiation of tumors in situ, followed by the tumor cell surviving fraction determined from colony counts in: (A) the lung. (B) the spleen. (C) petri dishes. (D) jejunal crypts.

C As the name implies, the in vivo-in vitro assay begins with an in vivo component where the tumor is irradiated in situ, followed by an in vitro component where the tumor is excised, the cells separated, and plated into petri dishes for a standard clonogenic assay. The other answer choices refer to colony assays performed in vivo only, with no in vitro component.

What tumor is MOST LIKELY to change size during radiation therapy? (A) Meningioma (B) Oligodendroglioma (C) Craniopharyngioma (D) Brain metastasis from breast cancer

C Craniopharyngiomas associated cysts can increase in size during radiation therapy. The other tumors listed are less likely to shrink or grow during fractionated treatment. References: Pediatric Radiation Oncology 5th edition, 2011.

According to the Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0, a Grade 3 radiation dermatitis consists of: (A) moist desquamation in skin folds. (B) moist desquamation with bleeding induced by minor trauma. (C) brisk erythema with extensive dry desquamation. (D) skin necrosis.

C Grade 1-*Faint erythema* or dry desquamation Grade 2-Moderate to brisk erythema; patchy moist desquamation, mostly *confined to skin folds and creases*; moderate edema Grade 3-Moist desquamation in areas *other than skin folds and creases*; bleeding induced by *minor trauma or abrasion* Grade 4-*Life-threatening consequences; skin necrosis or ulceration of full thickness dermis*; spontaneous bleeding from involved site; skin graft indicated. Grade 5-Death Reference:https://ctep.cancer.gov/protocolDevelopment/electronic_applications/docs/CTCAE_v5_Quick_Reference_8.5x11.pdf

Ipilimumab is a targeted agent that acts synergistically with radiotherapy by: (A) sensitizing tumor cells to radiotherapy by inhibiting DNA repair pathways. (B) sensitizing tumor cells to radiotherapy by inhibiting cytotoxic T cell lymphocytes. (C) increasing the immune response to tumor antigens released after radiotherapy. (D) increasing radiotherapy effectiveness by improving tumor oxygenation

C Ipilimumab (Yervoy) is a monoclonal antibody that works by enhancing T-cell activity by modifying the Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) inhibitory receptor. The activated T-cells increase the immune response to antigen-presenting tumor cells. Radiotherapy can increase the immune response even further by releasing tumor cell-associated antigens into the microenvironment.

How often are mutations caused by ionizing radiation passed on to the next generation? (A) Frequently (B) Sometimes (C) Rarely (D) Never

C Mutations caused by ionizing radiation are rarely passed on to the next generation, because first, these mutations must occur in germ cells; second, the particular germ cell (or its less mature precursor) affected must be the one selected to be fertilized; and finally, the mutation must not be of the type that causes embryonic/fetal lethality.

Which molecular marker is associated with increased radiosensitivity of cancer cells? (A) Loss of p53 (B) Amplification of KRAS (C) Overexpression of p16 (D) Nuclear localization of HIF-1β

C Overexpressed p16 can be a marker of an HPV infection, which in turn is associated with tumor radiosensitivity. Patients, especially those with lung and GI cancers, who have KRAS gene mutations or amplification have worse outcomes and exhibit resistance to some cancer treatments. HIF-1β is one of the subunits of the HIF-1 transcription factor and is constitutively bound to DNA, so as such, is always localized to the nucleus. Finally, loss of the p53 tumor suppressor protein frequently increases cellular radioresistance, not radiosensitivty.

For a patient to develop the gastrointestinal syndrome from radiation exposure, but not the cerebrovascular syndrome, s/he received an acute dose in the range of: (A) 0.1 - 1 Sv. (B) 1.1 - 5.9 Sv. (C) 6 - 25 Sv. (D) >25 Sv.

C The gastrointestinal syndrome develops after *acute doses of 8-10 Sv,* although some of the signs and symptoms of the syndrome are noted after doses as low as 6 Sv. The *threshold dose for the cerebrovascular syndrome is typically quoted as greater than 25 Sv*, although different published sources can differ widely

The skin reaction assay can be used to construct a radiation dose response curve by: (A) removing skin cells from an irradiated animal and transferring them to an organ in an untreated animal where colony formation occurs. (B) removing skin cells from an irradiated animal and plating them in a Petri dish for colony formation. (C) measuring the severity of the skin reaction in the irradiated animal using an ordinal scoring system. (D) monitoring the regrowth of nodules in irradiated areas of the animal's skin.

C The skin reaction assay is an example of a "functional" assay whereby a dose response curve is constructed that measures changes in the severity of the effect following irradiation. *In the case of the skin reaction assay, reactions such as erythema and desquamation are observed and given an arbitrary score as to their severity.* Average scores can be plotted as a function of dose. These types of experiments were used routinely in early studies measuring the effects of fractionation.

The oxygen-regulated protein lysyl oxidase (LOX) is involved in: (A) sprouting angiogenesis. (B) cell death by autophagy. (C) invasion and metastasis. (D) cytotoxic T-cell activation.

C Lysyl oxidases are extra cellular copper enzymes that catalyze the cross-linking of collagen and elastin in the extracellular matrix (ECM). This facilitates the detachment ofcellsfromtheECM,aswellascellmotilityandmigration. Incancercells,upregulationoftheHIF-1 target gene LOXL2 in response to tumor hypoxia enhances invasion and metastasis. Increased LOX expression is associated with disease progression, metastasis, and poor overall survival in breast cancer and head and neck cancers.

When γ-rays ionize water, the immediate products are: (A) e-aq. (B) OH- + H•. (C) H2O+ + e-. (D) H3O+ + OH•.

C. Gamma photons ionize water by removing an electron from the molecule. This leaves a water ion radical (H2O+) plus a fast electron (an ion pair). The water ion radical rapidly reacts with other water molecules to produce a hydronium ion (H3O+) and a hydroxyl radical (OH•). The electron released in the first reaction can bind to a water molecule forming an aqueous electron (e-aq). This in turn can react with additional water molecules to from hydrogen radicals (H•) and hydroxyl ions (OH-) H2O+ + e-. -->H3O+ + OH•-->e-aq-->OH- + H•.

Less than 40 Gy delivered to lymphatic drainage areas likely containing subclinical (microscopic) carcinoma: (A) does not reduce the nodal failure rate and will increase normal tissue toxicity. (B) does not reduce the nodal failure rate, even when adding concurrent chemotherapy. (C) will reduce the nodal failure rate, but not as much as total doses > 45 Gy. (D) does not reduce the nodal failure rate, even with addition of neoadjuvant chemotherapy.

C. A total dose of 45-50 Gy is commonly used for the treatment of subclinical disease and usually reduces the incidence of nodal failure by more than 90%. Using less than 45 Gy is still effective, but associated with a lower probability of control of nodal disease. The addition of chemotherapy before or during radiation therapy may increase nodal control, but still not as much as increasing the radiation dose instead. In addition, a possible drawback of adding chemotherapy is that it may trigger accelerated tumor stem cell repopulation.

For a course of spinal cord irradiation with 35 Gy in 2.5 Gy fractions 2 years prior, a 50% recovery of spinal cord tolerance is assumed. For re-irradiation, which dose fractionation scheme would be the safest? (Assume a cumulative, maximally tolerated dose of 25 x 2 Gy per fraction to 50 Gy, and an α/β ratio of 3 Gy.) (A) 21 Gy in 3 daily fractions (B) 27 Gy in 9 daily fractions (C) 30 Gy in 15 daily fractions (D) 36 Gy in 24 twice daily fractions

C. Fifteen fractions of 2 Gy QD to a total dose of 30 Gy would be the safest option in this case. EQD2=N*dx*(d+α/β)/(2+α/β) First, calculate the equivalent dose in 2 Gy fractions (EQD2, based on an α/β ratio of 3 Gy), for the original treatment course of 35 Gy using 2.5 Gy per fraction. The EQD2/3 = 14*2.5(2.5+3)/(2+3)=35*1.1=38.5 Gy. Then, assuming 50% tolerance dose recovery, the retreatment dose should not exceed 50 Gy - 19.25 Gy = 30.75 Gy (in 2 Gy fractions) (remaining tolerance of spinal cord) Answer option C is the closest, that is, 30 Gy in 15 fractions of 2 Gy.

Which histological staining technique is used to visualize senescent cells in vitro? (A) Ki-67 (B) MPM-2 (C) SA-β-galactosidase (D) Hematoxylin and eosin

C. Senescent cells develop a characteristic flattened and enlarged morphology ("fried eggs"), and can be identified more conclusively by staining with senescence- associated β-galactosidase (SA-β-gal) activity, which stains the perinuclear compartment blue. H&E staining is used to visualize the basic structure of tissue samples, KI-67 staining is used as a cell proliferation marker, MPM-2 is a mitosis-related stain

In a Petri dish where 1,000 human fibroblasts were plated and irradiated with 4 Gy of γ- rays, 5 colonies developed. In an unirradiated dish, 100 cells were plated and 10 colonies developed. What is the surviving fraction of the irradiated cells? (A) 0.001 (B) 0.005 (C) 0.05 (D) 0.1

C. Surviving fraction = [(number colonies counted) ÷ (the number of cells plated) x (plating efficiency)], where the plating efficiency = (number colonies counted) ÷ (the number of cells plated) for the unirradiated cells

Which protein is involved in the G2 phase cell cycle checkpoint, but NOT in the G1 phase cell cycle checkpoint? (A) pRb (B) p21 (C) Cdk1 (D) RAD50

C. The Cdk1 ("cyclin dependent kinase 1") protein, along with its partner Cyclin B, form a complex that phosphorylates target proteins and ultimately results in cell cycle progression from G2 phase into mitosis. G2 arrest occurs when the activity of the Cdk1/Cyclin B complex is inhibited, typically by upstream inhibition of the Cdc25C phosphatase. Rb protein (pRb) is responsible for a major G1 checkpoint, blocking S-phase entry and cell growth p21- cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1,(CDKN1A)-21 represents a major target of p53 activity and thus is associated with linking DNA damage to cell cycle arrest at multiple stages. RAD50-The Mre11/Rad50/Nbs1 (MRN) complex is a regulator of cell cycle checkpoints and DNA repair. Defects in MRN can lead to defective S-phase arrest when cells are damaged.

Tumor cells develop multi-drug resistance to chemotherapeutics by: (A) silencing genes associated with drug metabolism. (B) upregulating genes that code for DNA repair proteins. (C) overexpressing P-glycoprotein on their outer cell membranes. (D) increasing hepatic detoxification and clearance of the chemotherapy drug(s).

C. Tumor cells develop multi-drug resistance to chemotherapeutics by upregulating the MDR1 gene that codes for the energy-dependent, integral membrane transporter P-glycoprotein. The role of P-glycoprotein is to increase the efflux of chemotherapy drugs out of the cell, in particular, drugs derived from natural products (e.g., plant toxins).

TBI to an absorbed dose equivalent of 3-4 Sv would MOST likely cause which of the following? (A) Severe diarrhea within 8 hours (B) Neutropenic fever within 3 hours (C) Nausea and vomiting within 2 hours (D) Dry or moist skin desquamation within 24 hours

C. The prodromal radiation syndrome causes a dose-dependent increase in the frequency of nausea/vomiting, fatigue, headaches, fever, and skin reddening. Skin desquamation and neutropenia would take days/weeks to develop, not hours. Finally, doses in the 3-4 Sv range are generally too low to cause early onset, severe diarrhea in exposed individual

Which of the following catalyzes strand exchange during homologous recombination repair of DNA double strand breaks? (A) Artemis (B) BRCA1 (C) RAD51 (D) RAD52

C. RAD51 is a human homologue of the RecA recombinase found in E. coli and mediates the invasion of the homologous strand of the sister chromatid. RAD52 provides protection against exonucleolytic degradation. BRCA1 assists in the loading of RAD51. Artemis removes 3' and 5' overhangs during non-homologous end joining.

Which of the following compounds requires hypoxia for its anti-cancer activity? (A) Paclitaxel (B) Pemetrexed (C) Temozolomide (D) Tirapazamine

D *Tirapazamine exhibits selective cytotoxicity for hypoxic cells*. In the presence of hypoxia, *tirapazamine is metabolized to form a highly reactive radical species capable of inducing single- and double-strand breaks. Under aerobic conditions* the radical is back oxidized to the parent compound. It is this futile cycling that provides hypoxic selectivity.

Of the following types of radiation, which will cause the greatest amount of cell killing and DNA damage per unit dose? (A) 60Co gamma rays (B) 6 MeV electrons (C) 250 MeV protons (D) 250 MeV/n carbon ions

D 250 MeV/n carbon ions are considered a medium linear energy transfer (LET) radiation quality (the other three types are considered low LET radiation qualities) and will generate the greatest degree of DNA damage and cell killing per unit dose. The clinical relative biological effectiveness of carbon ions compared to 60Co gamma rays is approximately 3.

Which is the CORRECT match between whole body dose and likely outcome? (A) 2 Sv - epilation within a week (B) 9 Sv - brain edema within minutes (C) 5 Sv - low red blood cell count within a month (D) 7 Sv - nausea, vomiting, and diarrhea within hours

D A dose of 7 Sv will cause severe nausea and vomiting as well as diarrhea, typically within a couple of hours of radiation exposure. Doses greater than 8 Gy are usually lethal (although there have been exceptions), but not because of brain edema, but rather because of the gastrointestinal syndrome. The cerebrovascular syndrome, which would cause brain edema as a major symptom, has a much higher threshold dose than 9 Gy. A dose of 5 Gy is a high enough dose to elicit the hematopoietic syndrome; however red blood cell counts are typically the least affected by radiation exposure. A dose of 2 Gy is at or near the threshold dose to cause epilation, but even if it did occur, it would take longer than a week to manifest.

Compared to the mycotoxin aflatoxin B1, the neoplastic transformation frequency for an equitoxic dose of X-rays is: (A) much higher. (B) higher. (C) lower. (D) much lower.

D Aflatoxin B1, a mycotoxin produced by Aspergillus fungi, is arguably the most potent carcinogens known. In neoplastic transformation assays in vitro, it produces a much higher (by up to two orders of magnitude depending on the experimental system) transformation frequency than an equitoxic dose of X-rays. This demonstrates that, contrary to popular belief, ionizing radiation is not a particularly potent carcinogen. References: Chapter 3 in Tannock, Hill, Bristow and Harrington. The Basic Science of Oncology, Fourth Edition, 2005.

Telomere degradation with each subsequent cell division ultimately will result in: (A) necrosis. (B) apoptosis. (C) autophagy. (D) senescence

D As a cell divides, its telomeres are reduced in size until the sequence is no longer able to "cap" the end of the chromosome. Once this occurs, the end of the chromosome may be recognized as an open break end, akin to an unrejoined DNA double stranded break, at which point the cellular repair machinery permanently halts cell cycle progression, causing the cell to senesce

Which of the following molecules is the target for hyperthermia-induced cell killing? (A) DNA (B) RNA (C) Lipid (D) Protein

D Even at mild hyperthermic temperatures, *proteins are prone to unfolding and losing their function*, whereas DNA, RNA and lipids are more resistant to heat effects.

When comparing a traditional chemotherapy agent to a nanoparticle formulation of the same agent, all of the following properties will change, EXCEPT: (A) clearance rate. (B) biodistribution. (C) circulating time. (D) mechanism of action.

D Nanoparticles can be used as drug delivery vehicles for traditional chemotherapy agents. Nanoparticle formulations are advantageous in terms of changing the drug's biodistribution, increasing its circulation time, and changing its clearance rate. However, nanoparticles do not change the drug's mechanism of action.

Genetic deficiency in which of the following components of the NHEJ machinery results in severe combined immunodeficiency (SCID) in humans? (A) ATM (B) RPA (C) Rad50 (D) Artemis

D Non-homologous end-joining (NHEJ) through V(D)J recombination is a critical pathway to provide genetic diversity in Tcell receptors and immunoglobulins. The RAG nucleases create hairpins in V, D, and J segments that are cleaved by Artemis before end-joining occurs. Artemis deficiency therefore results in severe combined immunodeficiency due to defective V(D)J recombination. Genetic deficiency of ATM results in ataxia telangiectasia but not SCID. The RPA protein binds single stranded DNA and plays an important role in homologous recombination (HR), but not NHEJ. Deletion of RPA is embryonic lethal in mice, as is the deletion of RAD50, which is involved in both HR and NHEJ.

Which of the following proteins or protein complexes are common to both the NHEJ and HR DNA double-strand break repair pathways? (A) DNA-PK complex (B) Rad51 (C) Rad52 (D) Mre11-Rad50-Nbs1 complex

D The Mre11-Rad50-Nbs1 (MRN) complex is responsible for damage recognition and end resection in both pathways. The DNA-PK complex is involved in the NHEJ Rad51 facilitates strand exchange and Rad52 provides protection against exonucleolytic degradation during HR. The two major pathways for repair of DNA double-strand breaks (DSBs) are homologous recombination (HR) and nonhomologous end joining (NHEJ). HR leads to accurate repair, while NHEJ is intrinsically mutagenic. NHEJ is a relatively simple DSB repair pathway (Figure 1). Both ends of the break are first bound by the Ku70/Ku80 heterodimer, which then recruits the catalytic subunit of the DNA dependent protein kinase (DNA-PKcs). If necessary, the ends can be trimmed by nucleases (such as Artemis) or filled in by DNA polymerases (such as Polμ or Polλ) to create compatible ends. Finally, the ligation complex, consisting of DNA ligase IV, X-ray cross-complementation group 4 (XRCC4) and Xrcc4 like factor (XLF)/Cernunnos ligates the ends [4, 5]. NHEJ can take place throughout the cell cycle. For an extensive review on NHEJ HR uses a sequence similar or identical to the broken DNA as a template for accurate repair. The sister chromatid is used as an identical template in the S and G2 phases of the cell cycle, when the DNA has been replicated. BRCA2 mediates the replacement of RPA by RAD51, to form a nucleoprotein filament that searches for the homologous sequence on the sister chromatid. After strand invasion, catalyzed by RAD51 and many other proteins, the DNA end is extended using the intact sequence as a template.

The mutation in chromosome 13 found in patients with retinoblastoma leads to: (A) decreased retinal maturation. (B) increased cholesterol deposition. (C) increased tumor promoter activity. (D) decreased tumor suppressor activity.

D The RB1 gene located on chromosome 13 is a tumor suppressor gene which codes for a protein that controls cell growth and division. It also stops other proteins from triggering DNA replication.

The Ku proteins involved in NHEJ are: (A) DNA ligases. (B) protein kinases. (C) metalloproteases. (D) DNA binding proteins.

D The eukaryotic Ku protein is a heterodimer of Ku70 (XRCC6) and Ku80 (XRCC5) that participates in NHEJ, although is not a DNA repair protein per se. Instead, Ku functions as a molecular scaffold to which other proteins involved in NHEJ can bind, including the main repair protein, DNA-PKcs. Ku binds to and forms a sheath around the DNA that is capable of sliding along the helix as repair progresses. It also acts to stabilize and align broken DNA ends so that they don't unwind or mis-rejoin. The two major pathways for repair of DNA double-strand breaks (DSBs) are homologous recombination (HR) and nonhomologous end joining (NHEJ). HR leads to accurate repair, while NHEJ is intrinsically mutagenic. NHEJ is a relatively simple DSB repair pathway (Figure 1). Both ends of the break are first bound by the Ku70/Ku80 heterodimer, which then recruits the catalytic subunit of the DNA dependent protein kinase (DNA-PKcs). If necessary, the ends can be trimmed by nucleases (such as Artemis) or filled in by DNA polymerases (such as Polμ or Polλ) to create compatible ends. Finally, the ligation complex, consisting of DNA ligase IV, X-ray cross-complementation group 4 (XRCC4) and Xrcc4 like factor (XLF)/Cernunnos ligates the ends [4, 5]. NHEJ can take place throughout the cell cycle. For an extensive review on NHEJ HR uses a sequence similar or identical to the broken DNA as a template for accurate repair. The sister chromatid is used as an identical template in the S and G2 phases of the cell cycle, when the DNA has been replicated. BRCA2 mediates the replacement of RPA by RAD51, to form a nucleoprotein filament that searches for the homologous sequence on the sister chromatid. After strand invasion, catalyzed by RAD51 and many other proteins, the DNA end is extended using the intact sequence as a template.

In the absence of stem cell transplantation, approximately how long would it take for half of a group of individuals exposed to 4 Sv whole-body irradiation to die of the hematopoietic syndrome? (A) 1 week (B) 2 weeks (C) 1 month (D) 2 months

D The hematopoetic syndrome is elicited by acute, whole body radiation doses of more than about 2 Sv. The human LD50/60 in the absence of stem cell support is in the range of 3-4 Sv.

Using the linear-quadratic model, what mathematical expression corresponds to the slope of the survival curve? (A) α (B) β (C) (α+β) (D) (α+βD)

D The linear quadratic survival expression is S = e^-(α^D+ β^D^2), where S is the cell surviving fraction, D is the dose delivered and the terms α and β are constants particular to the cell type. Taking the natural log of both sides of the equation "linearizes" it into the general form of *"y = mx + b", with a slope corresponding to α + βD.*

Which of the following statements concerning post-irradiation gene expression profiling of tissues is true? (A) There is minimal inter-individual variation in post-irradiation gene expression profiles. (B) Post-irradiation gene expression profiles show a high degree of correlation with post-irradiation proteomic profiles. (C) The patterns of gene expression post-irradiation remain relatively constant over time. (D) The patterns of gene expression change over time and vary between individuals.

D The overarching conclusion from ionizing radiation gene expression studies in human cells and tissues is that gene expression profiles show significant time dependence, dependence upon the tissue being analyzed, significant inter-individual variation, and minimal correlation to post-irradiation proteomic profiles. Begg AC. Predicting response to radiotherapy: evolutions and revolutions. Int J Radiat Bio 2009; 85(10):825-836

The time interval between radiation therapy and clinical manifestation of injury to slow or non-proliferating normal tissues can be as long as: (A) 2 years. (B) 5 years. (C) 10 years. (D) >10 years.

D There is no evidence to suggest that the incidence of late normal tissue complications reaches a plateau. Patients can be at risk for certain complications even > 10 years after exposure.

Japanese survivors of the atomic bombings who were irradiated in utero with 1 Sv between the 8th and 15th weeks of gestation have a probability of severe mental retardation of approximately: (A) 0.05. (B) 0.1. (C) 0.2. (D) 0.4.

D Severe mental retardation was not observed to be induced by radiation *before 8 weeks after conception* or *after 25 weeks*. The *most sensitive period is 8 to 15 weeks after conception;* for exposure during *weeks 16 to 25, the risk is four times smaller.* The relationship appears to be linear, and the data are consistent with a *probability of occurrence of mental retardation of 40% at a dose of 1 Gy.* A dose threshold is also consistent with the presumed deterministic nature of mental retardation that would require the killing of a minimum number of cells to be manifest. ICRP Publication 90 (2003) offers the view that the induction of severe mental retardation during this most sensitive period has a threshold of *at least 0.3 Gy, with the absence of risk at lower doses*.

Which of the following types of DNA damage is NOT caused by exposure to ionizing radiation or to alkylating agents? (A) Monoadducts (B) Single strand breaks (C) Interstrand crosslinks (D) Pyrimidine dimers

D Ionizing radiation and alkylating agents share in common the generation of: -DNA monoadducts, -single strand breaks and -interstrand crosslinks. Cyclobutane dimers are formed by exposure to ultraviolet UVC radiation

When should gemcitabine be administered to achieve its maximum radiosensitizing effect? (A) At the conclusion of radiotherapy (B) Simultaneous with each radiation dose (C) Within a few hours after each radiation dose (D) Within a few hours before each radiation dose

D. Based on both laboratory and clinical studies, there is no evidence of gemcitabine radiosensitization when irradiation occurred prior to drug exposure; the greatest sensitizer enhancement ratio was observed when gemcitabine was administered for 24 h before irradiation. Gemcitabine is an analog of cytarabine (cytosine arabinoside) which acts both as a cytotoxin against human cancers, particularly pancreatic and non-small cell lung cancer, as well as a potent radiosensitizer.

Mutations which render the DNA mutation repair mammalian pathway MOST sensitive to ionizing radiation are: (A) mismatch repair (MMR). (B) base excision repair (BER). (C) nucleotide excision repair (NER). (D) non-homologous end-joining (NHEJ).

D. NHEJ mutants (e.g., Ku70/80 deficient, LIG4 deficient, etc.) are deficient in the repair of DNA double strand breaks, the most critical type of DNA lesion induced by ionizing radiation. BER, NER and MMR mutants either display wild-type radiosensitivity or are only slightly radiosensitive.

How does reducing the dose rate of low LET radiation affect the survival curve parameters α and β? (A) α is increased and β is reduced. (B) α is reduced and β is increased. (C) α is reduced and β is unchanged. (D) α is unchanged and β is reduced.

D. The theoretical underpinning of the α/β survival model is the theory of dual radiation action, that postulates that lethal lesions result from the combination of two sublesions. In one case, both lesions result from the same energy absorption event, in this example, a single electron track. These single track events have linear kinetics and are proportional to dose (αD). In another case, each sublesion results from an independent track. These multi-track events produce curvature and are proportional to the square of the dose (βD2). By reducing the dose rate, α is unaffected since both sublesons originate from the same track. β, on the other hand; is reduced since each sublesion is formed from an independent track and as the dose rate is lowered, the probability that a specific sublesion might be repaired before a second sublesion that it can potentially combine with is formed is increased. This reduces the frequency of multi-track events and cell survival is increased.

As defined by the ICRU, exposure is a measure of: (A) charged particle dose in a known volume of air. (B) photon dose produced in a known volume of air. (C) ionization by photons produced in a known volume of water. (D) ionization by charged particles produced by photons in a known volume of air.

D. Understanding the limitation of exposure measurements is important. *Exposure and roentgen is only defined for x and gamma ray radiation*. Exposure is the measured of the *total charge in air (ionization)* of all electrons and positrons liberated by photons in a known volume of air.

Regarding the HIF-1 transcription factor: (A) HIF-1α down-regulates genes associated with cell motility, invasion, and metastasis. (B) HIF-1α must be stabilized by hydroxylation before it can act as a transcription factor. (C) HIF-1β must be transported from the cytoplasm to the nucleus. (D) reoxygenation induces its activity, causing production of pro-angiogenic cytokines.

D. In addition to its role as an oxygen sensor and regulator of angiogenesis, HIF-1 has also been implicated in tumor radioresistance. HIF-1 is activated in response to other cellular stressors besides hypoxia, including an excess of reactive oxygen species as would be produced by tumor reoxygenation that occurs after irradiation. This, in turn, leads to the activation of angiogenic cytokines that help protect tumor vasculature - vascular endothelial cells in particular - from radiation injury.