CMD Radiation Physics & Radiobiology 2 — Questions and Answers
Question 1: Which effect describes the increased radiosensitivity of cells in the presence of molecular oxygen?
- Oxygen effect (Correct answer)
- Bystander effect
- Fractionation effect
- Repair effect
Correct answer: Oxygen effect
The oxygen effect refers to the 2–3 fold increase in radiation sensitivity when cells are well-oxygenated compared to hypoxic conditions, because oxygen fixes radiation-induced free radical damage.
Question 2: In conventional fractionation for radiation therapy in the US, the standard daily fraction size is:
- 1.8–2.0 Gy (Correct answer)
- 1.0–1.2 Gy
- 3.0–5.0 Gy
- 0.5–0.8 Gy
Correct answer: 1.8–2.0 Gy
Conventional fractionation uses 1.8–2.0 Gy per fraction, delivered once daily, five days per week, balancing tumor control with normal tissue sparing.
Question 3: Which quantity describes the rate of energy deposition by a charged particle per unit path length?
- Linear Energy Transfer (LET) (Correct answer)
- Specific Activity
- Fluence Rate
- Kerma
Correct answer: Linear Energy Transfer (LET)
LET (keV/μm) describes the energy deposited per unit length along a charged particle's track and determines the biological effectiveness of different radiation types.
Question 4: The 'Four Rs of Radiobiology' include repair, reassortment, repopulation, and:
- Reoxygenation (Correct answer)
- Redistribution only
- Regression
- Radioresistance
Correct answer: Reoxygenation
Reoxygenation is the fourth R, where hypoxic tumor cells become oxygenated between fractions as adjacent cells die, increasing overall radiosensitivity.
Question 5: Which cell cycle phase is most radiosensitive?
- M phase (mitosis) and G2 phase (Correct answer)
- S phase (late)
- G1 phase (early)
- G0 phase
Correct answer: M phase (mitosis) and G2 phase
Cells are most radiosensitive in M and late G2 phases because DNA repair mechanisms are suppressed and chromosomal aberrations are most lethal at this stage.
Question 6: Tissue weighting factors (wT) in radiation protection are used to calculate:
- Effective dose (Correct answer)
- Absorbed dose
- Kerma
- Dose equivalent only
Correct answer: Effective dose
Tissue weighting factors account for the relative radiosensitivity of different organs and are used with equivalent dose to calculate effective dose (measured in Sieverts).
CMD Radiation Physics & Radiobiology 2
Which effect describes the increased radiosensitivity of cells in the presence of molecular oxygen?