LET & Hyperthermia

The yield of hydrogen peroxide from the radiolysis of water is higher for high LET radiation than for low LET radiation because:

the higher density of free radicals produced in high LET tracks facilitates radical-radical interactions.

The type of electromagnetic energy used for hyperthermia treatment is:

microwave Both microwave and RF radiations have been used for hyperthermia.

Cells are most sensitive to heat during which portion of the cell cycle?

S Hyperthermia effects proteins. The S phase is most radioresistant, but heat sensitive. NHEJ occurs in G1. HRR occurs in S/G2.

What is the LET for carbon ions in the spread out bragg peak?

50 keV/µM

Approximately what percent oxygen concentration yields cellular radiosensitivity halfway between the fully aerobic and fully anoxic response?

0.5% or 3 mm Hg.

A cell line is irradiated with 6 MeV x-rays under both aerobic and severely hypoxic conditions; the OER defined from the slopes of the aerobic and hypoxic survival curves is found to be 3.2. Approximately what OER would you expect to see if the same cell were irradiated with 15 MeV neutrons using the same protocols>

1.6 The OER decreases as the LET increases, falling to 1.0 at very high LETs. Neutrons are intermediate in ionization density and this have intermediate LET values and therefore an OER typically about 1.6.

How many ion clusters are formed by 55 keV/μm silicon ion along a 1 μm- segment of the ion trajectory through the cell nucleus? Assume silicon ion irradiation with the beam parallel to a cellular monolayer and that ion clusters are uniformely spaced along the silicon ion track.

500 clusters every 1 μm

For many biological endpoints, the linear energy transfer (LET) at which the relative biological effectiveness (RBE) reaches its highest value is:

100 keV/μm. The LET represents the rate of energy loss per unit path length in collisions in which energy is "locally" absorbed, rather than being carried away by energetic secondary particles. At a LET of approximately 100 keV/μm, the average density of ionization events along an incident particle's track roughly corresponds to the diameter of the DNA double helix, meaning a higher probability of a double-strand break being produced than at either lower or higher LET.

Treatment at 42°C for 25 minutes, 44°C for 4 minutes and 45°C for 1 minute will cause approximately the same level of cell killing as treatment at 43°C for how many minutes?

17

A cell line irradiated with x-rays under aerobic conditions is well fit by a survival curve for which n = 3 and D₀ = 1.5 Gy. Which of the following survival curve parameters would best apply under the irradiation and oxygenation conditions indicated?

2.5 MeV α-particles at p0₂ of 1 mm Hg; n = 1 and D₀ = 0.5 Gy α-particles have a survival curve that is characterized by a small n and D₀ even under hypoxic conditions. The D₀ is expected to increase for x-rays under hypoxic conditions. High-energy protons have similar biologic properties to x-rays.

What is the LET for argon ions in the spread out bragg peak?

250 keV/µM

Which of the following corresponds to the temperature and duration of hyperthermia treatment?

41-45°C, for 30-60 minutes Both microwave and radiofrequency energy are used to deliver hyperthermia treatment.

The break point in an Arrhenius plot is observed at approximately;

43°C The Arrhenius relationship has been used to define the temperature dependence of the rate of cell killing, which can then be used as a method for thermal dosimetry. This is done by plotting the log of the slope (1/Do) of cell survival curves as a function of temperature. Typically, Arrhenius plots have a biphasic curve and the point at which the slope changes is referred to as a "break point." Above the break point for nearly all cell types, a change in temperature of 1°C will double the rate of cell killing. Below the break point, the rate of cell killing for rodent cell lines drops by a factor of 4 to 8 for every drop in temperature of 1°C. The change in slope below the break point is caused by the development of thermotolerance during heating. Arrhenius plot slopes for human cell lines are well described and quite uniform across a variety of cell types. The break point for human cells is near 43.5°C

What type of ionizing radiation has the highest LET?

75 MeV/nucleon argon ions Clinically relevant 75 MeV per nucleon argon ions have LET 250 keV/μm. 1 GeV/nucleon and 18 MeV/nucleon carbon ions have LET values of approximately 10 keV/μm and 108 keV/μm, respectively. 2.5 MeV alpha particles have an LET value of approximately 170 keV/μm. 150 MeV protons are considered low LET, with values in the range of 0.5 keV/μm.

¹³¹I tositumomab (Bexxar) is:

A radiolabeled antibody against the CD20 antigen over-expressed in non- Hodgkin's lymphoma cells ¹³¹I tositumomab (Bexxar) is a radiolabeled antibody against the CD20 cell surface antigen found in a very high percentage of B cell non-Hodgkin's lymphomas. The beta- and gamma- emitting (not alpha-emitting) radioisotope ¹³¹I is used for treatment of thyroid cancer, and is administered singly, not attached to any antibody. The primary clinical toxicity from ¹³¹I tositumomab is a dose-related, reversible, hematopoietic suppression.

What is the effect on both RBE and the α/β ratio as the LET for the type of radiation increases up to 100 keV/μm?

Both increase As the LET for different forms of radiation increases to about 100 keV/μm, both the RBE and the α/β ratio for the corresponding cell survival curves increase due primarily to an increase in the α parameter.

Which cellular or tissue process is reduced or eliminated following high LET radiation?

Cell cycle age response The variation in cellular radiosensitivity with cell cycle position or "age" is reduced or eliminated for high LET radiation. The variation in radiosensitivity as a function of cell age is qualitatively similar for neutrons and x-rays; that is, with both types of radiation, maximum sensitivity is noted at or close to mitosis, and maximum resistance is evident late in S phase. There is, however, a quantitative difference in that the range of radiosensitivity between the most resistant and the most sensitive phases of the cell cycle is much less for fast neutrons than for x-rays. As LET increases, the variation in radiosensitivity through the cell cycle decreases, so that at very high LET, the age-response function is almost flat—that is, radiosensitivity varies little with the phase of the cell cycle.

What is the definition of relative biological effectiveness (RBE) of a test radiation, r, compared with x-rays?

D250 / Dr D250 is the dose of 250 keV photons requires to have a particular biologic effect. Sometimes, this is just referred to as Dx, or a reference radiation dose required to have a biological effect. Dr is the dose of the test radiation required to have the same biologic effect. Therefore, a higher relative biological effectiveness (RBE) for the test radiation will correlate with a higher D250 / Dr.

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?

DNA helix 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 a given isoeffect survival level, how does the OER change as the LET increases from 10 - 100 keV/micron?

Decreases

Equal doses of densely- and sparsely- ionizing radiations, such as 1-MeV α-particles and 1-MeV γ-rays, produce different levels of biological damage. Which of the following is the most plausible explanation of the difference in biological effectiveness?

Densely ionizing radiations produces more ionizations per unit length of the track

The optimal time to deliver heat (relative to radiation) in order to achieve the greatest radiosensitization is:

During RT The greatest heat radiosensitization is produced when the heat is delivered as close to the time of irradiation as possible, since a likely mechanism for the sensitizing effect is heat denaturation of the proteins (enzymes) associated with the repair of radiation damage.

Initial shoulder of hyperthermic survival curve suggests the repair of sublethal damage, T/F?

False Due to the difference in the mode of action, it is important not to draw conclusions for heat based on the interpretation of radiation dose-response curves. The amount of energy involved in cell inactivation is a thousand times greater for heat than for x-rays.

LET is inversely proportional to mass of the particle moving through a medium, T/F?

False LET does not directly dependent on mass of the particle, but rather indirectly through kinetic energy relationships (i.e., the rest mass of the particle and its velocity).

Long duration mild hyperthermia (42°C) cannot inhibit the repair of sublethal radiation damage (SRD), because of the low temperature, T/F?

False Long duration mild hyperthermia is as good as 43°C incubation in eliminating the repair of RT-induced SRD, which is particularly true in a low dose-rate for brachytherapy.

Radiofrequency ablation combined with radiotherapy produces radiosensitization, T/F?

False Radiofrequency ablation is accomplished by inserting a RF probe into or near a tumor mass, and then heating it to temperatures that produce frank tissue necrosis. RF ablation is typically used singly, not simultaneously with radiation therapy.

Which of the following medically-useful ionizing radiation types is densely ionizing?

Gamma rays from ¹²⁵I decay The ¹²⁵I decay mechanism is electron capture to form the nearly-stable tellurium-125. This is followed by gamma decay at 35 keV and Auger electron emission at 50-500 eV. LET values for these low energy gamma rays and Auger electrons is greater than 30 keV/μm. Energy of gamma rays from ⁶⁰Co decay and gamma rays produced in the annihilation reaction is in a 1-MeV range and LET ~ 0.2 keV/μm. ⁹⁰Y is a high-energy beta-emitting isotope. The maximum energy of the beta particles is 2.27 MeV with a mean of 0.93 MeV. These electrons are sparsely ionizing.

The carbon ion RBE for hypoxic cells compared with that for aerated cells is:

Greater The carbon ion RBE is the dose required to produce a certain effect in X-irradiated cells divided by the carbon ion dose to produce the same biological effect. This ratio will be greater for cells irradiated under hypoxic conditions because of the much greater dose required to produce the effect in the X-irradiated cells where an oxygen effect is present, compared to the high LET irradiated cells where the oxygen effect is absent.

Cells in which phase of the cell cycle are most resistant to hyperthermia-induced cell killing?

G₁ the S phase is most sensitive to hyperthermia because a lot of synthesis and proteins are involved in the S phase, so heat is able to denature them. G₁ is most resistant to hyperthermia-induced cell killing. Hyperthermia is most effective radiosensitizer if given at the time of radiation. However, the best therapeutic effective is achieved using sequential heat treatment rather than simultaneous treatment. Sequential treatment spares normal tissue injury because of differences in blood flow between tumor and normal tissues.

Based on an understanding of thermal dose and the effects of timing between heat and radiation, which of the following protocols would be expected to produce the greatest thermal enhancement ratio?

Heat concurrent with radiation For the combination of heat and radiation, the greatest thermal enhancement ratio (TER) occurs when the two agents are given simultaneously. For moderately elevated temperatures that do not in and of themselves cause much heat-induced cell killing, hyperthermia has been shown to interfere with the repair of radiation-induced DNA damage and results in a reduction in the shoulder on the radiation survival curve when the two agents are administered either simultaneously or within minutes of each other.

What mediates tumor cell kill when the boron containing drug (¹⁰B enriched sodium borocaptate, Na₂B₁₂H₁₁SH) is irradiated with low energy neutrons?

High LET alpha particles The production of high LET alpha particles and 7Li occurs when a drug containing boron (such as BSH) is irradiated with low energy neutrons during boron neutron capture therapy. The short path range of the high LET alpha particles cause local irradiation and provide some tumor selectivity for boron containing drugs provided the drugs can be selectively delivered/localized to the tumor.

Hyperthermia combined with radiation may be effective in cancer therapy because:

Hypoxic tumor cells, which may be at a low pH and nutritionally-deprived, exhibit enhanced sensitivity to heat Tumor cells are not intrinsically more sensitive to heat than normal cells. Heat does not affect the number of ionizations produced by a given dose of radiation.

Relative to a single dose exposure, how is the RBE of neutrons affected by fractionation in comparison to RBE of fractionated photons?

Increases but to lesser extent The RBE of high LET radiation increases with fractionation, because the shoulder region is low after single or fractionated exposures from high LET, indicating little sparing. The alpha component of cell kill dominates, and death more an exponential component of dose. After exposure to x-rays, the shoulder region of the survival curve remains after each fraction and is higher than after exposure to neutrons, and thus the ability to repair sublethal damage is higher. This results in higher RBE for neutrons after fractionated exposures. The RBE would not decrease with fractionation, even if the LET of the high LET radiation was 100keV/μm - in fact one would expect RBE to be highest for neutrons at that LET.

What happens to the relative biological effectiveness (RBE) as the linear energy transfer (LET) increases?

Increases then decreases RBE increases with increasing LET to a maximum of 100 keV/µM and decreases thereafter.

What is the effect of oxygen on cell killing produced by high LET heavy-charged particles?

Little change High LET radiation does not depend on free radicals/oxygen.

About ⅔ of biological damage after low LET irradiation is caused by:

OH· - the hydroxyl free radical. OH· - the hydroxyl free radical are produced by secondary reactions in radiolysis of water. OH•, the hydroxyl free radical, diffuses in tissue within a cylinder about 4nm in diameter (about 2x diameter of a DNA double helix) and causes about ⅔ of all biological damage: half-live - 10⁻⁹ seconds.

Thermal Enhancement Ratio (TER) is defined as:

RT dose without heat / RT dose for equivalent effect with heat

What radiobiological processes contribute to the inverse dose-rate effect?

Redistribution The inverse dose rate effect is where decreasing the dose rate results in increased cell killing. Decreasing the dose rate over the range from 1 Gy/min to 0.01 Gy min generally results in an increase in the surviving fraction following irradiation with a specific dose of radiation due to repair of sublethal damage. The inverse dose-rate effect is the observation that, as the dose rate declines further over a critical range, cellular survival decreases as the same constant dose is delivered. This effect relates to reassortment of cells into the radiosensitive phase of the cell cycle through the DNA damage-induced G₂ block.

The wavelength of 915 MHz microwaves used in the treatment of hyperthermia is approximately how many centimeters (cm)?

Requires knowledge of the speed of light, c = 3⋅10⁸ m/s and the 915 MHz is 9.15⋅10⁸s⁻¹; wavelength = c/frequency = 3⋅10⁸/9.15⋅10⁸ = 0.328 m = 33 cm.

For low LET radiations, asymmetrical exchange-type aberrations (such as dicentric or ring) lead to a loss of reproductive integrity. The effect of irradiation of induction of aberrations and survival can be approximated by the two-component linear quadratic model. At low doses, what does the linear term in the linear quadratic model describe?

Single directly induced DNA double-strand break The single event component of the LQ survival curve occurs predominantly at low doses.

What is the linear attenuation coefficient (µ)?

The interaction probability per unit length during x-ray interaction with matter. µ=dI/(Idx), where I is the intensity, dx is the matter thickness.

As the LET increases for a series of radiations to which cells are exposed, what is the impact on the alpha and beta cell cycle parameters?

The α parameter primarily increases As the LET increases, it becomes increasingly αD killing and the sensitivity of the cells increase. The β parameter remains relatively constant.

Nanoparticle-mediated hyperthermia sensitizes tumors to radiation therapy via:

Tumor stem cell sensitization Hyperthermia using gold nanoshells results in preferential sensitization of tumor stem cells.

Upon heating, the heat shock transcription factor, HSF1, stimulates production of heat shock proteins, T/F?

True

Increasing the LET increases the number of ionizations along a particle track while reducing the number of tracks required to produce a specific dose, T/F?

True 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 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.

It may take as long as a week after hyperthermia treatment for thermotolerant cells to revert to their normal heat sensitivity, T/F?

True At temperatures of 39 - 42°C, thermotolerance can occur during heating, but it does not persist for days thereafter. Thermotolerance is a substantial effect, where the slope of the heat survival curve can decrease by a factor of 4 - 10 fold. Because of this, use of hyperthermia in the clinic is generally limited to once, or at the most twice, per week.

Once a photon transfers all its energy to an electron, the LET is that of the electron, T/F?

True Photons, such as 250 KV X-rays, in passing through tissue produce no ionizations directly but only by setting in motion atomic electrons of tissue molecules. Electrons set in motion by incident photons have a broad energy distribution which is dissipated in tracts with LET ranging from about 0.4 to 40 keV/μm. Radiation therapy high energy photons can generate neutrons with energy between 0.1 to 2 MeV through photon interactions with nuclei of high atomic number materials that constitute the linac head and collimator systems. These neutrons in passing through tissue also produce no ionization directly but by setting protons in motion by knock on collisions with hydrogen nuclei of the cellular water molecules. Protons set in motion by photoneutrons dissipate energy over a range of LET up to about 90 keV/μm.

Cells maintained in a low pH microenvironment tend to be more sensitive to heat than cells maintained at physiologic pH, T/F?

True Tissues maintained under conditions of low pH tend to be sensitive to heat. G₂ cells are quite radiosensitive, but somewhat more resistant to heat killing, comparatively speaking. It is the chronically hypoxic cells in tumors (that typically exist in acidic microenvironments) that tend to be more sensitive to heat than acutely hypoxic cells.

A primary advantage of HDR brachytherapy for the treatment of prostate cancer is that:

Tumor response should be improved by using larger fraction sizes because of the lower α/β ratio associated with prostate cancer compared with that for the surrounding normal tissues Most clinical evidence now indicates that prostate cancers have unusually low α/β ratios, possibly as low as 1.5 Gy, and significantly less than the α/β ratio of roughly 3 Gy assumed for late complications in the normal tissues surrounding the prostate. This low α/β ratio suggests that prostate tumors should be especially sensitive to the large fraction sizes used for HDR brachytherapy. Since the OER usually increases with dose and dose rate, it would be expected to be greater for HDR than LDR brachytherapy.

Particle selection for high LET radiation therapy should be determined by the particle's ability to:

achieve a high RBE and a low OER. Biologically speaking, the goal of high LET radiation therapy is to maximize the RBE and minimize the OER. In terms of ionizations per cell, the goal is not necessarily to maximize the number of them, but rather to ensure that the ionizations that do occur are optimally spaced to coincide with the diameter of the DNA helix, which increases the likelihood of producing lethal DNA double strand breaks from the passage of a single particle track.

Linear energy transfer (LET) is defined as the average:

amount of energy imparted to a medium by a charged particle per unit track length. this is usually expressed in terms of keV/μm. It is important to remember that this is an average quantity and the amount of energy imparted along microscopic segments of the track of equal length is highly variable.

The average linear energy transfer (LET) for a proton therapy beam is _____.

highest at the distal edge of the spread-out Bragg peak (SOBP) The LET of a proton increases as it loses energy and slows down near the Bragg peak region. The LET of a proton reaches its maximum in the fall-off region of a Bragg peak.

The induction of DNA DSBs at high LET follows a _____ pattern as a function of dose.

linear With higher LET, the induction of DSBs follows a linear pattern as a function of dose. Unrepaired DSBs correlate best with cell killing.

For 80 keV/μm carbon ions, the dose response curve for dicentric chromosome aberrations is:

linear and steep. 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.

A linear-quadratic dose response for dicentric formation will be produced by:

low LET radiation delivered at a high dose rate. A dicentric chromosome aberration can be produced when two chromosome breaks are formed in two different chromosomes, and these breaks are in close enough proximity to one another with respect to space and time that an improper exchange occurs. For high LET radiations, both breaks are likely to be produced along the same charged particle track, resulting in a linear dose response, regardless of dose rate. For low LET radiations, while some exchanges can be produced by single particle tracks, an increasing number are produced by two independent tracks as the dose increases, yielding a linear-quadratic dose response. At a low dose rate, most dicentrics result from single track events. Two track exchanges are reduced because the tracks are spaced out temporally, allowing time for one of the lesions potentially involved in the dicentric to be repaired before the second lesion can be produced. This results in a linear dose response.

The carbon ion RBE is highest when:

many small radiation doses are used.

A brief exposure of cells to hyperthermic temperatures of 45°C immediately prior to x-irradiation will result in:

minimal change in the shoulder region of the radiation survival curve. Hyperthermic temperatures around 45°C produce little change in the shoulder region of an x-ray survival curve, but will result in a steeper final slope (smaller value for D₀).

How does the frequency of clustered lesions relate to LET?

proportional Clusters are referred to spurs, blobs or short tracks. So when these damages overlap they result in a multiply damaged site or a clustered lesion. Clustered lesions increase with LET. More spurs occur with LOW let radiations, whereas high LET produces more blobs and tracks.

Heat-shock proteins reduce both heat-induced cell killing and radiosensitization because they:

reduce heat-induced protein damage. HSPs function as intracellular molecular chaperones for other proteins, and as such, play important roles in protein-protein interactions and the prevention of potentially damaging protein aggregation. By helping to stabilize partially unfolded or aggregated proteins, HSPs reduce the damage caused by hyperthermia, and in turn, reduce both heat-induced cell killing and radiosensitization.

Biological advantages of high LET radiation include:

reduced OER, reduced repair, reduced cell cycle differential and a higher RBE for slowly cycling tumors.

How will reducing the dose rate of low LET radiations from 1000 cGy per minute to 0.5 cGy per minute change the α-β parameters in the linear quadratic survival model?

α will be unchanged while β decreases. The alpha-beta survival model is based on the Theory of Dual Radiation Action. This theory postulates the pairwise interaction of spatially and temporally proximate sublesions to form the lethal lesions that ultimately kill the cell. A fraction of these lethal lesions form when both sublesions result from a single energy adsorption event (e.g. single electron track). This process produces the linear (alpha) component of the dose response. Alpha type events are dose rate independent and dominate at low doses and dose rates. The quadratic portion of the dose response (Beta) results from sublesions that are formed by independent events (multiple electron tracks). Since the number of sublesions themselves is directly proportional to radiation dose, reducing the dose rate spreads out the production of sublesions over time allowing a fraction of them to be repaired before the formation of the second proximate sublesions required for lethal lesion creation occurs. By lowering the dose rate, alpha (single-hit) events remain constant while beta (multi-hit) events decrease.