ASTRO RadBio 2017-XXXX Qs - LET

??????Which statement concerning the linear energy transfer (LET) is CORRECT? A. LET is equal to the energy transferred by ionizing radiation to soft tissue per unit mass of soft tissue. B. LET is equal to the number of ion pairs formed per unit track length C. Once a photon transfers all its energy to an electron, the LET is that of the electron. D. LET is the quotient of the average energy that a particle lost in causing ionization to the average distance it travels between two consecutive ionizations. E. The track average method and the energy average method for calculating LET give different numerical values for therapy protons in soft tissue.

????

Which of the following pairs of radiation type and approximate LET value is CORRECT? A. 150 MeV protons - 0.5 keV/m B. 1 GeV Fe ions - 20 keV/m C. 60Co gamma-rays - 15 keV/m D. 2.5 MeV alpha-particles - 5 keV/m E. 250 kV X-rays - 10 keV/m

A. 150 MeV protons have an LET of approximately 0.5 keV/m. LET values: 1 GeV Fe ions -- 143, 60Co gamma-rays --0.6, 2.5 MeV alpha-particles -- 166 and 250 kV X-rays -- 2

Which of the following statements concerning LET is FALSE? a. The highest RBE occurs for radiations with LET values of approximately 100 keV/m b. High LET radiations yield survival curves with low D0 values c. The OER increases with increasing LET d. High LET radiations often produce exponential survival curves e. LET is an average energy (in keV) transferred from a charged particle traversing a distance of 1 μm in the medium

C. OER decreases with increasing values of LET. Maximum effectiveness and therefore RBE reaches a peak for radiations whose LET is approximately 100 keV/µm. The RBE of high LET radiations is generally high, resulting in low values for D0. The survival curves resulting from irradiation of cells with high LET radiations are typically exponential. LET is the term that describes the density of ionization or the average amount of energy lost (in keV) to the medium per unit of track length (μm).

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 uniformly spaced along the silicon ion track. A. 0.5 cluster every 1 μm or 1 cluster every 2 μm. B. 5.5 clusters every 1 μm C. 500 clusters every 1 μm D. 5,500 clusters every 1 μm. E. 55,000 clusters every 1 μm

C. On the average, the formation of a three-ion cluster requires dissipation of 110 eV.

The carbon ion RBE for hypoxic cells compared with that for aerated cells is: a. Equal b. Lower c. Greater d. Dependent upon the endpoint being measured e. The same as the OER

C. 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. The oxygen enhancement ratio, OER, for carbon ions would be lower than that for low LET radiations, but the absolute value of the OER is not related to the value of the RBE.

Which of the following statements concerning LET is INCORRECT? A. LET is proportional to charge density of a medium. B. LET is proportional to charge (squared) on the particle moving through a medium. C. LET is inversely proportional to speed (squared) of the particle. D. LET is inversely proportional to mass of the particle moving through a medium. E. LET is related to density of ionizations along the particle's track.

D. LET does not directly dependent on mass of the particle, but rather indirectly through kinetic energy relationships. In mathematical terms, Restricted linear energy transfer is defined by L Δ = d E Δ /d x where d E Δ is the energy loss of the charged particle due to electronic collisions while traversing a distance d x , excluding all secondary electrons with kinetic energies larger than Δ. If Δ tends toward infinity, then there are no electrons with larger energy, and the linear energy transfer becomes the unrestricted linear energy transfer which is identical to the linear electronic stopping power. Here, the use of the term "infinity" is not to be taken literally; it simply means that no energy transfers, however large, are excluded.

Concerning RBE, OER and LET, which of the following statements is TRUE? A. Maximum cell killing per dose delivered occurs at an LET corresponding to approximately 1000 keV/µm B. RBE changes the most over the LET range of 0.1 to 10 keV/µm C. The relationship between OER and LET is bell-shaped D. RBE decreases with increasing LET above about 100 keV/µm E. OER increases with LET

D. RBE decreases with increasing LET above about 100 keV/µm. This is thought to be due to the "overkill" effect in which many more ionizations (and damage) are produced in a cell traversed by a very high LET particle than are minimally necessary to kill it, thereby "wasting" some of the energy. Maximum cell killing occurs at an LET of approximately 100 keV/µm, not 1000 keV/µm. RBE shows the greatest changes for LET values between roughly 20 and 100 keV/µm. OER decreases slowly with increasing LET for low LET values, but falls rapidly after LET exceeds about 60 keV/µm and, therefore, does not follow a bell-shaped curve.

Which of the following statements is correct? Compared with damage from low LET radiation, damage from high LET radiation: A. Is reduced to a greater extent in the presence of sulfhydryl compounds B. Shows more potentially lethal damage recovery C. Exhibits a greater OER D. Is less subject to split-dose recovery E. Shows greater sparing when the irradiation is given at a low dose rate

D. There is little or no split-dose recovery following high LET radiation exposure because the single dose survival curves for high LET radiations have little or no shoulder. There is also little or no potentially lethal damage recovery, oxygen effect or radioprotection afforded by the presence of sulphydryl compounds. Delivery of a radiation dose at a low dose rate leads to less sparing for a high LET radiation compared with a low LET radiation

Which of the following medically-useful ionizing radiation types is densely ionizing? a. Gamma rays from Iodine-125 decay b. Gamma rays from Cobalt-60 decay c. Gamma rays produced in the annihilation reaction d. Protons used in radiation therapy e. Beta particles emitted by Yttrium-90

A. Note, that as the energy of a given particle increases its density of ionization decreases. Protons that are used form therapy are considered to be similar in that regard to phonons from linac. The iodine-125 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 Cobalt-60 decay and gamma rays produced in the annihilation reaction is in a 1-MeV range and LET ~ 0.2 keV/μm. The proton used for XRT has 10 MeV energy and considered intermediate in ionizing density. The protons formed from fission decay are densely ionizing. Yttrium-90 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.

Which of the following statements is correct about Relative Biological Effect (RBE)? a. RBE is the ratio of doses of two different radiations that produce the same survival fractions b. RBE is the ratio of survival fractions produced by the same doses of two different radiations c. Beta-particles have higher RBE values than alpha-particles d. High LET radiation have lower RBE values than low LET radiation e. Stereotactic ablative radiotherapy has a higher RBE than conventional radiotherapy

A. RBE is calculated based on the ratio of doses, not the ratio of effect. Generally, high LET radiations have higher the RBE values. Stereotactic ablative radiotherapy involves high dose per fraction radiotherapy, compared to conventional radiotherapy, and does not necessarily use radiation of different energies.

What is the effect on both RBE and the alpha/betaratio as the LET for the type of radiation increases up to 100 keV/m? a. Both remain the same b. Both increase c. Both decrease d. The RBE decreases while the alpha/beta increases e. The RBE increases while the alpha/beta decreases

B. As the LET for different forms of radiation increases to about 100 keV/m, both the RBE and the alpha/beta ratio for the corresponding cell survival curves increase due primarily to an alpha increase in the alphaparameter.

Which of the following types of ionizing radiation has the highest LET? a. 2.5 MeV alpha particles b. 75 MeV/nucleon argon ions c. 1 GeV/nucleon carbon ion d. 18 MeV/nucleon carbon ions e. 150 MeV protons

B. 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.

Which of the following statements concerning RBE is TRUE? The RBE: A. Is lower for neutrons than for protons over the therapeutic energy range B. For high LET particles is greater for hypoxic cells than for oxygenated cells of the same type C. For carbon ions is diminished when delivered in several fractions rather than as a single dose D. For heavy charged particles is greatest at the beginning of the particle tracks

B. The RBE for high LET particles is greater for hypoxic cells than for well-oxygenated cells of the same type because there is little or no oxygen effect for high LET radiation. The RBE is greater for neutrons than it is for protons in the therapeutic energy range because the high energy protons used in radiotherapy are of a relatively low LET and therefore possess an RBE of approximately 1.1. The RBE for carbon ions, or any other type of high LET radiation, is greater for a fractionated irradiation compared with an acute exposure because of the substantial sparing exhibited with the reference X-rays with fractionation. The RBE for charged particles is low at the beginning of the particle track and greatest near the end of the track, in the Bragg peak region. RBE does show a fractionation dependence; it decreases with increasing fraction size.

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? a. Sparsely-ionizing radiation is less effective due to wastage of ionizations in water b. Densely-ionizing radiation produces more ionization per unit mass c. Densely-ionizing radiations tend to deposit more energy than is needed to produce the effect d. The ratio of DNA double- to single-strand breaks is approximately 20:1 per 1 Gy for sparsely ionizing radiation and approximately 1:1 for densely ionizing radiation, independent of radiation quality e. Densely ionizing radiations produces more ionizations per unit length of the track

E. Equal doses produce the same number of ionizations per unit mass, regardless of radiation quality. 1 Gy = 1 J/kg which corresponds to 2x10^17 ionizations/kg, roughly 10^6 ionizations per mammalian cell. Biological efficiency of ionizing radiation is related to its spatial energy concentration along the track, which is the random localized pattern of the individual ionizing produced by a charged particle or secondary electrons set in motion by the original particle. For fast electrons set in motion by γ-rays, the energy releases are widely spaced and, even though the track passes through DNA, there is a chance that no energy will be released in it. On the other hand, the track left by α-particle is so dense that if it passes through DNA, there will be enough energy released in it to destroy the ability of the DNA to function properly. The LET value of cobalt- 60 γ-rays is 0.25 keV/μm. This means that along each micron of track length, 250 eV is released by 1 MeV γ- ray, which would be enough to produce 7-8 ion pairs/μm. In contrast, 1 MeV α-particle (LET = 250 keV/μm) will lose ~ 1000 times as much energy along each micron of track and may produce 7000-8000 ion pairs/μm. 7