Radiation Biology
What to know
Core topics: the 4 R's (repair, reassortment, repopulation, reoxygenation), the linear-quadratic (LQ) model, α/β ratios, cell survival curves, and the biologic basis of fractionation.
Concepts to master
- α/β ratio describes fractionation sensitivity. Early-responding tissues and tumors ~10 Gy; late-responding tissues ~3 Gy. Low α/β (e.g., prostate ~1.5) supports hypofractionation.
- BED = nd(1 + d/(α/β)) lets you compare schedules.
- Oxygen effect / reoxygenation explains why fractionation spares tumors less than late tissues.
- Repair of sublethal damage between fractions protects late tissues.
- Relative biologic effectiveness (RBE) and LET for particle/high-LET therapy.
Traps
- Forgetting that late effects are more fraction-size sensitive; large fractions increase late toxicity.
- Misapplying LQ at very large doses per fraction (LQ breaks down).
- Confusing repopulation (accelerated proliferation) with reassortment.
Rule of thumb
Smaller fractions protect late tissues more than tumors because late tissues have low α/β. When a stem asks why hyperfractionation reduces late toxicity, the answer is differential repair/reassortment favoring late tissue sparing.
Worked example
Compare 60 Gy in 30 fractions (2 Gy) vs 60 Gy in 20 fractions (3 Gy) for late tissue (α/β = 3).
- 2 Gy schedule: BED = 30×2×(1+2/3) = 100 Gy₃
- 3 Gy schedule: BED = 20×3×(1+3/3) = 120 Gy₃ Higher BED for late tissue at 3 Gy/fraction → greater late toxicity risk.