Radiation and Cancer Biology
Core concepts
Master the 4 R's (repair, reassortment, repopulation, reoxygenation) and how fractionation exploits them. Know the linear-quadratic model: survival = e^−(αD+βD²), the α/β ratio, and how BED = nd(1 + d/(α/β)) converts schedules. Understand DNA damage and repair (HR/NHEJ), the oxygen effect (OER ~2.5–3), and cell-cycle radiosensitivity (M most sensitive, late S most resistant). Know clinically relevant α/β values: ~10 Gy for tumors/early effects, ~3 Gy for late effects. Recall dose-rate effects, RBE for protons/heavy ions, and the hallmarks relevant to radiosensitizers.
Traps
Questions blur early vs late effects and which α/β applies. Remember: low α/β (late) is more sensitive to fraction size-hypofractionation worsens late toxicity. Don't confuse OER with RBE. Repopulation kick-in (accelerated proliferation) argues for shortened overall time in head/neck and cervix. BED comparisons must match α/β; mixing values invalidates the comparison.
Rule of thumb
BED = nd(1 + d/(α/β)). To compare two schedules, compute BED with the same α/β for the tissue of concern.
Worked example
Compare 60 Gy/30 fx (2 Gy) vs 40 Gy/15 fx (2.67 Gy) for late effects (α/β = 3). Schedule A: 60 × (1 + 2/3) = 100 Gy₃. Schedule B: 40 × (1 + 2.67/3) ≈ 75.6 Gy₃. Despite lower total dose, B's larger fraction size raises late-effect BED relative to its tumor BED-illustrating why fraction size matters for late tissue.