RT Radiation Biology 1 — Questions and Answers
Question 1: Which of the following correctly lists the 4 R's of radiobiology?
- Repair, Reoxygenation, Redistribution, Repopulation (Correct answer)
- Reduction, Resistance, Regeneration, Recovery
- Repair, Reduction, Reoxygenation, Resistance
- Redistribution, Resistance, Recovery, Repopulation
Correct answer: Repair, Reoxygenation, Redistribution, Repopulation
The 4 R's of radiobiology are Repair, Reoxygenation, Redistribution, and Repopulation, which describe how cells and tissues respond between fractions of radiation.
Question 2: Which phase of the cell cycle is considered MOST radiosensitive?
- Late S phase
- G1 phase
- M phase (mitosis) (Correct answer)
- Early S phase
Correct answer: M phase (mitosis)
M phase (mitosis) and the G2/M boundary are the most radiosensitive parts of the cell cycle because DNA is condensed and cells are actively dividing.
Question 3: What is the oxygen enhancement ratio (OER) for high-LET radiation such as fast neutrons?
- Approximately 1.0 (no oxygen enhancement) (Correct answer)
- Approximately 2.5–3.0
- Approximately 1.5
- Approximately 3.5
Correct answer: Approximately 1.0 (no oxygen enhancement)
High-LET radiation has an OER close to 1.0, meaning oxygen does not significantly enhance radiation damage because the dense ionization tracks cause direct DNA damage regardless of oxygen.
Question 4: What is the primary purpose of the linear-quadratic (LQ) model in radiation therapy?
- Calculating tumor volume from imaging
- Predicting cell survival and comparing fractionation schedules (Correct answer)
- Measuring radiation output from a linac
- Determining beam energy selection
Correct answer: Predicting cell survival and comparing fractionation schedules
The LQ model (S = e^(–αD – βD²)) is used to predict cell survival after irradiation and to compare the biological equivalence of different fractionation schedules using the BED formula.
Question 5: Which cell cycle phase is considered MOST radioresistant?
- M phase
- G2 phase
- Late S phase (Correct answer)
- G1 phase
Correct answer: Late S phase
Late S phase is the most radioresistant phase of the cell cycle because cells are actively synthesizing DNA and have increased repair capacity through homologous recombination.
Question 6: What does the alpha/beta ratio describe in radiobiology?
- The ratio of high-LET to low-LET radiation used in treatment
- The fractionation sensitivity of tissues and tumors (Correct answer)
- The oxygen enhancement ratio for a given tissue
- The relative biological effectiveness of a radiation type
Correct answer: The fractionation sensitivity of tissues and tumors
The alpha/beta ratio (in Gy) describes how sensitive a tissue is to changes in fraction size; tissues with low α/β ratios (e.g., spinal cord ~2 Gy) are more sensitive to large fractions than tissues with high α/β ratios (e.g., tumors ~10 Gy).
Question 7: Which type of ionizing radiation has the HIGHEST linear energy transfer (LET)?
- X-rays (photons)
- Electrons
- Gamma rays
- Alpha particles (Correct answer)
Correct answer: Alpha particles
Alpha particles have the highest LET because they are large, doubly charged particles that interact intensely with matter and deposit energy very densely along their short track.
RT Radiation Biology 1
Which of the following correctly lists the 4 R's of radiobiology?