Radiation Physics & Radiobiology Flashcards

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

Answer: Gray (Gy)

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

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

Answer: Relative Biological Effectiveness (RBE)

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.

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

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

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.

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

Answer: 2–3 Gy

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.

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

Answer: Compton scattering

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.

The depth of maximum dose (dmax) for a 6 MV photon beam in water is approximately:

Answer: 1.5 cm

For a 6 MV photon beam, dmax occurs at approximately 1.5 cm depth in water due to the build-up region created by secondary electron scatter.