Chapter 7
Linear energy transfer and relative biological effectiveness

1. For a given radiation type, the density of ionization:

a. increases with the velocity of the ionizing particle.

b. decreases with the velocity of the ionizing particle.

c. is independent of the velocity of the ionizing particle.

2. Which of the following statements is true?

a. Linear energy transfer (LET) is the energy transferred to the biologic material per unit mass of the material.

b. LET is the quotient dE/dl, where dE is the energy that a particle lost in causing an ionization and dl is the distance that the ionizing particle travels between two ionizations.

c. LET is the quotient dE/dl, where dE is the average energy locally imparted to the medium by a charged particle when the particle has traversed a distance dl.

3. The "track average" method and the "energy average" method for calculating LET give different numerical values in the case of:

a. x-rays and gamma rays.

b. protons.

c. alpha particles.

d. neutrons.

4. Which of the following statements is true?

a. LET is used to describe the quality of different types of radiation.

b. The higher the LET value, the lower the biologic effectiveness of the radiation.

c. The LET value of 60Co gamma rays is 5 keV/micrometer.

d. The LET value of 10 MeV protons is 0.2 keV/micrometer.

5. Which of the following statements is true?

a. Equal doses of different types of radiation have the same biologic effect.

b. The "relative biologic effectiveness" (RBE) is used to compare different types of radiation.

c. In the definition of RBE, the reference radiation is 1 MeV gamma rays.

d. The RBE of a radiation r is equal to Dr/D250, where Dr and D250 are the doses of the radiation r and of 250 kV x-rays, respectively, that produce the same biologic effect.

6. If the LD50 for 250 kV x-rays is 6 Gy and the LD50 of a neutron beam is 4 Gy, the RBE of the neutron beam is equal to 1.5.

a. True.

b. False.

7. As the dose decreases, the RBE of a given radiation type:

a. increases.

b. decreases.

c. remains constant.

8. Fractionation introduces a "waste in dose", which is more pronounced for beams with a wide shoulder than for beams with a narrow shoulder in the survival curve.

a. True.

b. False.

9. The RBE of a given beam does not depend on:

a. the total dose delivered to the tissue.

b. the number of fractions in which the dose is delivered to the tissue (in the case of a fractionated regimen).

c. the dose-rate (in the case of continuous irradiation).

d. the type of irradiated tissue.

e. the endpoint studied.

f. the phase of the cell cycle in which the irradiated cells are at the moment the irradiation begins.

g. the radiation quality (LET).

10. The RBE varies with increasing LET as follows:

a. it increases slowly at low LET values and more rapidly at high LET values up to 500 keV/micrometer.

b. it increases with LET values up to 100 keV/micrometer and subsequently decreases with increasing LET.

c. it is a constant function of LET.

11. There is an optimal LET value for the production of a biologic effect because:

a. lower LET values involve "wasting" of radiation energy.

b. this LET value corresponds to an average separation between ionizing events which is approximately equal to the diameter of the DNA molecule.

c. radiations with higher LET values have a low probability of producing a double-strand break by the passage of a single charged particle.

12. RBE values are low for tissues that accumulate and repair a substantial amount of sublethal damage and high for tissues that do not.

a. True.

b. False.

13. The relation between OER and LET is as follows:

a. OER is a constant fuction of LET.

b. OER has a value of about 3 at high LET values and then decreases to zero at low LET values.

c. OER has a value of about 3 at low LET values and then decreases to unity at high LET values (approximately 200 keV/micrometer).

14. An absorbed dose of 0.1 Gy of radiation with a radiation weighting factor of 20 corresponds to an equivalent dose of:

a. 0.2 Sv.

b. 2 Sv.

c. 20 Sv.