CMD Radiation Physics & Radiobiology 1 — Questions and Answers
Question 1: What is the unit of absorbed dose in the SI system used in medical dosimetry?
- Gray (Gy) (Correct answer)
- Rad
- Sievert (Sv)
- Roentgen (R)
Correct answer: Gray (Gy)
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?
- Relative Biological Effectiveness (RBE) (Correct answer)
- Oxygen Enhancement Ratio (OER)
- Linear Energy Transfer (LET)
- Therapeutic Ratio
Correct 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.
Question 3: The oxygen enhancement ratio (OER) is highest for which type of radiation?
- Low-LET radiation (X-rays, gamma rays) (Correct answer)
- Fast neutrons
- Alpha particles
- Heavy ions
Correct 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.
Question 4: In the linear-quadratic (LQ) model, the α/β ratio for late-responding normal tissues is typically:
- 2–3 Gy (Correct answer)
- 8–10 Gy
- 15–20 Gy
- 0.5–1 Gy
Correct 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.
Question 5: Which physical process is dominant for photon interactions in tissue at megavoltage energies used in radiation therapy?
- Compton scattering (Correct answer)
- Photoelectric effect
- Pair production
- Rayleigh scattering
Correct 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.
Question 6: The depth of maximum dose (dmax) for a 6 MV photon beam in water is approximately:
- 1.5 cm (Correct answer)
- 0.5 cm
- 3.0 cm
- 5.0 cm
Correct 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.
CMD Radiation Physics & Radiobiology 1
What is the unit of absorbed dose in the SI system used in medical dosimetry?