CMD Radiation Physics & Radiobiology

Question 1

What is the unit of absorbed dose in the SI system used in medical dosimetry?

Accepted Answer

Gray (Gy)

Answer

The Gray (Gy) is the SI unit of absorbed dose, equal to 1 joule of energy deposited per kilogram of tissue.

Question 2

Which concept describes the ability of ionizing radiation to cause DNA double-strand breaks relative to a reference radiation?

Accepted Answer

Relative Biological Effectiveness (RBE)

Answer

RBE compares the biological effectiveness of a given radiation type to a reference (usually 250 kVp X-rays or Co-60 gamma rays) in producing a specified biological endpoint.

Question 3

The oxygen enhancement ratio (OER) is highest for which type of radiation?

Accepted Answer

Low-LET radiation (X-rays, gamma rays)

Answer

Low-LET radiations such as X-rays and gamma rays have the highest OER (~2.5–3), meaning hypoxic cells are significantly more radioresistant compared to high-LET radiations.

Question 4

In the linear-quadratic (LQ) model, the α/β ratio for late-responding normal tissues is typically:

Accepted Answer

2–3 Gy

Answer

Late-responding normal tissues have low α/β ratios (approximately 2–3 Gy), making them more sensitive to fraction size changes than early-responding tissues.

Question 5

Which physical process is dominant for photon interactions in tissue at megavoltage energies used in radiation therapy?

Accepted Answer

Compton scattering

Answer

Compton scattering is the dominant photon interaction in tissue at megavoltage energies (1–10 MeV) because it is dependent on electron density rather than atomic number.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD Radiation Physics & Radiobiology