Radiation Oncology: Principles, Fractionation, and Safety
Core concepts
Radiation oncology questions test radiobiology, treatment planning, and toxicity recognition. Know the 4 R's of radiobiology: repair, redistribution, repopulation, and reoxygenation. Understand the linear-quadratic model conceptually-the α/β ratio separates early-responding tissues (high α/β, ~10 Gy) from late-responding tissues (low α/β, ~2–3 Gy)-and why fractionation spares late effects while still controlling tumor.
Treatment modalities
Distinguish definitive vs palliative vs adjuvant intent. Know external beam (teletherapy) versus brachytherapy (interstitial, intracavitary, surface/plesiotherapy). Understand dose constraints for critical organs (eye, spinal cord, lung, kidney, bowel) and the concept of tumor control probability vs normal tissue complication probability (TCP/NTCP).
Traps
- Confusing acute (early) vs late radiation effects; late effects are dose-per-fraction dependent.
- Assuming higher total dose always improves outcome without considering normal tissue limits.
- Overlooking radiosensitivity differences among tumor types.
- Forgetting palliative endpoints (pain relief, quality of life) are legitimate goals.
Worked rule of thumb
For a nasal tumor near the eye, remember: late-responding tissues (lens, retina, optic nerve) limit fraction size. If a plan requires high dose near the eye, reduce dose-per-fraction or accept a lower total dose to respect the constraint-TCP must be balanced against NTCP.
Safety and handling
Know time, distance, shielding for personnel protection, and the inverse square law. Familiarize yourself with radioactive iodine/brachytherapy handling and client/owner precautions.