RT Radiation Biology 2 — Questions and Answers
Question 1: Compared to low-LET radiation, high-LET radiation has a relative biological effectiveness (RBE) that is:
- Lower — less biologically effective per unit dose
- Equal — the same effectiveness per unit dose
- Higher — more biologically effective per unit dose (Correct answer)
- Variable — depends only on dose rate, not LET
Correct answer: Higher — more biologically effective per unit dose
High-LET radiation has a higher RBE because its dense ionization tracks cause more complex, clustered DNA damage that is more difficult to repair than the sparse ionization of low-LET photons.
Question 2: In the linear-quadratic model S = e^(–αD – βD²), the αD (linear) component represents:
- Two-hit (double-strand break) cell killing
- Single-hit (single-track) lethal cell killing (Correct answer)
- Sublethal damage repair
- Repopulation between fractions
Correct answer: Single-hit (single-track) lethal cell killing
The αD linear component represents single-track, single-hit lethal events where one ionizing particle event causes an irreparable, lethal lesion proportional to dose.
Question 3: What is 'reoxygenation' in the context of tumor radiobiology?
- Adding oxygen gas to radiation beams to increase ionization
- The re-establishment of oxygen supply to previously hypoxic tumor cells after irradiation (Correct answer)
- Breathing supplemental oxygen during radiation treatment sessions
- The use of hyperbaric oxygen chambers post-treatment
Correct answer: The re-establishment of oxygen supply to previously hypoxic tumor cells after irradiation
Reoxygenation occurs when previously hypoxic (radioresistant) tumor cells become oxygenated after a fraction kills well-oxygenated cells, making the surviving hypoxic cells more sensitive to subsequent fractions.
Question 4: Which tissues have a LOW alpha/beta ratio of approximately 2–3 Gy?
- Rapidly proliferating tumors (e.g., head and neck cancers)
- Acutely responding tissues like oral mucosa and skin
- Late-responding tissues such as spinal cord and lung (Correct answer)
- Hematopoietic (bone marrow) tissues
Correct answer: Late-responding tissues such as spinal cord and lung
Late-responding tissues like spinal cord, lung, and kidney have low α/β ratios (~2–3 Gy), meaning they are particularly sensitive to large fraction sizes and benefit more from standard fractionation.
Question 5: The Bergonié-Tribondeau law states that cells are MORE radiosensitive when they:
- Are highly differentiated with low mitotic activity
- Have high mitotic activity and are less differentiated (Correct answer)
- Have a high intracellular oxygen concentration
- Are in late S phase of the cell cycle
Correct answer: Have high mitotic activity and are less differentiated
The Bergonié-Tribondeau law (1906) states that cells with high mitotic activity, great proliferative future, and less differentiation are more radiosensitive — which is why cancer cells are generally more sensitive than mature normal cells.
Question 6: What is 'sublethal damage repair' (SLDR) in radiobiology?
- Permanent, irreversible damage that prevents cell division
- Repair of radiation-induced lesions that, if unrepaired, could accumulate into lethal damage (Correct answer)
- Damage only to RNA, not to DNA
- Repair processes that occur exclusively in tumor cells
Correct answer: Repair of radiation-induced lesions that, if unrepaired, could accumulate into lethal damage
SLDR refers to the repair of DNA lesions (e.g., single-strand breaks) that are individually non-lethal but become lethal if combined with additional damage; this repair is the biological basis for the shoulder on cell survival curves.
Question 7: What is the Tumor Control Probability (TCP) in radiation oncology?
- The probability that all normal tissue complications are avoided
- The statistical likelihood that a given radiation dose will eradicate all clonogenic tumor cells (Correct answer)
- The ratio of irradiated tumor volume to total tumor volume
- The percentage of patients who achieve partial response
Correct answer: The statistical likelihood that a given radiation dose will eradicate all clonogenic tumor cells
TCP is the probability that a specific radiation dose will sterilize every clonogenic (colony-forming) cell in the tumor, resulting in local control; it increases with dose in a sigmoidal relationship.
RT Radiation Biology 2
Compared to low-LET radiation, high-LET radiation has a relative biological effectiveness (RBE) that is: