FRCR Radiobiology 2 — Questions and Answers
Question 1: What is the standard dose per fraction in conventional radical radiotherapy?
- 1.2 Gy
- 1.8–2.0 Gy (Correct answer)
- 3.0 Gy
- 5.0 Gy
Correct answer: 1.8–2.0 Gy
Conventional fractionation uses 1.8–2.0 Gy per fraction, balancing tumor cell kill while allowing repair of sublethal damage in normal tissues.
Question 2: Which of the following best describes hyperfractionation in radiotherapy?
- Fewer fractions with a higher dose per fraction
- Treating 5 days per week with standard 2 Gy fractions
- Multiple smaller fractions per day to increase total dose while limiting late toxicity (Correct answer)
- Compressing treatment into fewer weeks with the same dose per fraction
Correct answer: Multiple smaller fractions per day to increase total dose while limiting late toxicity
Hyperfractionation uses multiple daily fractions with smaller doses per fraction, exploiting differential α/β ratios to increase total dose and improve tumor control while protecting late-responding tissues.
Question 3: A treatment delivers 2 Gy × 30 fractions to a tumor with α/β = 10 Gy. What is the Biologically Effective Dose (BED)?
- 60 Gy
- 72 Gy (Correct answer)
- 80 Gy
- 90 Gy
Correct answer: 72 Gy
BED = nd × (1 + d/[α/β]) = 30 × 2 × (1 + 2/10) = 60 × 1.2 = 72 Gy.
Question 4: Accelerated repopulation of tumor cells during radiotherapy is generally thought to begin after approximately:
- 1 week
- 2 weeks
- 3–4 weeks (Correct answer)
- 6–7 weeks
Correct answer: 3–4 weeks
Accelerated repopulation begins around 3–4 weeks into radiotherapy as surviving tumor cells increase their proliferative rate in response to radiation-induced cell loss.
Question 5: Prostate cancer has an unusually low α/β ratio estimated at approximately:
- 10 Gy
- 5 Gy
- 1.5 Gy (Correct answer)
- 8 Gy
Correct answer: 1.5 Gy
Prostate cancer has an α/β ratio of ~1.5 Gy, similar to late-responding normal tissues, making it particularly sensitive to high dose-per-fraction hypofractionated schedules.
Question 6: Stereotactic body radiotherapy (SBRT) is characterized by which fractionation schedule?
- 1.8–2 Gy × 25–30 fractions
- 1.2 Gy × 2 fractions per day
- 6–20 Gy × 3–5 fractions (Correct answer)
- 2 Gy × 15 fractions
Correct answer: 6–20 Gy × 3–5 fractions
SBRT uses ablative hypofractionated schedules of 3–5 large fractions (6–20 Gy each), delivering high doses with precision to minimize normal tissue exposure.
Question 7: Which statement correctly compares late-responding and early-responding tissues in fractionated radiotherapy?
- Late-responding tissues have higher α/β ratios and are more sensitive to high doses per fraction
- Early-responding tissues have lower α/β ratios and tolerate large fractions well
- Late-responding tissues have lower α/β ratios and are more sensitive to high doses per fraction (Correct answer)
- Both tissue types respond equally to changes in dose per fraction
Correct answer: Late-responding tissues have lower α/β ratios and are more sensitive to high doses per fraction
Late-responding tissues have lower α/β ratios (1–3 Gy), making them more sensitive to increased dose per fraction; conventional fractionation protects them by using small daily doses.
FRCR Radiobiology 2
What is the standard dose per fraction in conventional radical radiotherapy?