FRCR Radiobiology 1 — Questions and Answers
Question 1: According to the linear-quadratic model, what is the typical alpha/beta ratio for late-responding normal tissues?
- 1–3 Gy (Correct answer)
- 8–10 Gy
- 15–20 Gy
- 5–7 Gy
Correct answer: 1–3 Gy
Late-responding tissues have a low α/β ratio of ~1–3 Gy, making them more sensitive to high doses per fraction.
Question 2: Which phase of the cell cycle is most sensitive to ionizing radiation?
- Early S phase
- G1 phase
- G2/M phase (Correct answer)
- Late S phase
Correct answer: G2/M phase
G2/M phase cells have the highest radiosensitivity because they have limited time for DNA repair before cell division.
Question 3: What type of DNA damage is most responsible for radiation-induced cell death?
- Single-strand breaks
- Base modifications
- DNA-protein cross-links
- Double-strand breaks (Correct answer)
Correct answer: Double-strand breaks
Double-strand breaks are the most lethal form of radiation-induced DNA damage because they are difficult for cells to repair accurately.
Question 4: The oxygen enhancement ratio (OER) for low-LET radiation such as X-rays is approximately:
- 1.0
- 1.5
- 2.5–3.0 (Correct answer)
- 4.0–5.0
Correct answer: 2.5–3.0
The OER for X-rays is approximately 2.5–3.0, meaning hypoxic cells require 2.5–3 times more dose to achieve the same cell kill as well-oxygenated cells.
Question 5: Which of the following best defines the therapeutic ratio in radiotherapy?
- The ratio of alpha to beta values in the LQ model
- The ratio of tumor control dose to normal tissue tolerance dose
- The ratio of dose per fraction to total dose
- The difference between tumor control probability and normal tissue complication probability (Correct answer)
Correct answer: The difference between tumor control probability and normal tissue complication probability
The therapeutic ratio represents the balance between tumor control probability (TCP) and normal tissue complication probability (NTCP), guiding optimal treatment dose selection.
Question 6: According to the Bergonié and Tribondeau law, which cells are most radiosensitive?
- Highly differentiated, slowly dividing cells
- Rapidly dividing, undifferentiated cells (Correct answer)
- Post-mitotic cells with high metabolic activity
- Cells in G0 phase with low oxygen tension
Correct answer: Rapidly dividing, undifferentiated cells
Bergonié and Tribondeau established that cells are most radiosensitive when they are rapidly dividing, undifferentiated, and have a high mitotic rate.
Question 7: In the linear-quadratic (LQ) model, the linear (α) component of cell kill represents:
- Sublethal damage that can be repaired between fractions
- Cell kill proportional to the square of the dose
- Cell kill proportional to dose, representing lethal double-strand breaks from a single event (Correct answer)
- Cell kill occurring only at high dose rates
Correct answer: Cell kill proportional to dose, representing lethal double-strand breaks from a single event
The α (linear) component represents cell kill proportional to dose, corresponding to a single lethal event such as a non-repairable double-strand break from one radiation track.
FRCR Radiobiology 1
According to the linear-quadratic model, what is the typical alpha/beta ratio for late-responding normal tissues?