Radiation Oncology: Physics, Techniques, and Indications
What this domain tests
Physics fundamentals and how they translate into a prescription: dose definitions, photon interaction, fractionation biology, technique selection (3DCRT, IMRT/VMAT, SBRT, SRS, brachytherapy, protons) and the evidence-based indications for each. Calculation stems on BED and EQD2 are common.
Core rules
- Absorbed dose is energy per unit mass: 1 Gy = 1 J/kg = 100 cGy = 100 rad.
- Photon interactions: photoelectric effect dominates at low energy and scales with atomic number (contrast, bone); Compton scattering dominates at megavoltage energies and depends on electron density; pair production requires >1.022 MeV.
- Inverse square law: dose falls as 1/d². A source giving 200 cGy at 100 cm gives 50 cGy at 200 cm.
- Linear-quadratic model: BED = n·d × (1 + d/(α/β)). α/β ≈ 10 Gy for tumors and early-responding tissues, ≈ 3 Gy for late-responding tissues such as spinal cord and kidney. EQD2 = BED / (1 + 2/(α/β)).
- Technique logic: SBRT is ablative, few-fraction, steep-gradient treatment needing motion management; SRS is single-fraction intracranial treatment; protons deposit dose in a Bragg peak with no exit dose, valued in pediatric and skull-base disease.
- Normal-tissue constraints are conventionally quoted at 2 Gy per fraction: spinal cord 45–50 Gy, whole brain 45 Gy, lung V20 and mean dose limits, parotid mean dose <26 Gy.
Worked example
Compare 60 Gy in 30 fractions with 54 Gy in 3 fractions for a tumor (α/β = 10). Conventional: BED = 60 × (1 + 2/10) = 72 Gy₁₀. SBRT: d = 18 Gy, BED = 54 × (1 + 18/10) = 54 × 2.8 = 151.2 Gy₁₀. The SBRT course is far more potent biologically. Apply α/β = 3 to the adjacent late-responding tissue and the same schedule carries a much larger late-effect penalty - hence small volumes and strict constraints.
Traps
- Comparing BED values computed with different α/β, or trusting the LQ model above roughly 10 Gy per fraction.
- Quoting cord tolerance without stating fractionation.
- Confusing HDR with LDR brachytherapy dose rates, or ignoring iridium-192 decay (half-life ≈ 74 days).
- Assuming IMRT lowers dose everywhere - it redistributes dose, and low-dose spill can raise second-cancer risk.