Radiation Oncology
Core concepts
Understand radiobiology: the 4 Rs (repair, redistribution, repopulation, reoxygenation), the linear-quadratic model, α/β ratios (early-responding tumours ~10 Gy; late tissues ~3 Gy), and fractionation (conventional 1.8–2 Gy; hypofractionation; SBRT; hyperfractionation). Know dose constraints for organs at risk (spinal cord, brainstem, optic chiasm, lung V20, heart mean dose, parotid, bowel). Understand treatment planning (3D conformal, IMRT, VMAT, proton), image guidance (IGRT), and target volume definitions (GTV, CTV, PTV). Recognise acute vs late toxicity (mucositis, dermatitis, xerostomia, pneumonitis, fibrosis, second malignancy) and radiosensitisers (cisplatin in head/neck and cervix). Understand palliative radiotherapy roles (bone pain, bleeding, cord compression, brain metastases) and emergencies (SVC obstruction, spinal cord compression - dexamethasone + urgent RT).
Traps
Questions test fractionation logic (hypofractionation exploits lower α/β of late tissues; SBRT exploits high dose per fraction and steep gradients). Distinguish acute (days–weeks) from late (months–years) effects. Do not confuse GTV vs CTV vs PTV margins. Watch for dose-constraint questions where the OAR is the deciding factor.
Worked example / rule of thumb
A patient with bone metastasis and severe pain: single 8 Gy is equivalent to 30 Gy/10 fractions for pain control, with more re-treatment need but less burden. Rule of thumb: for palliation, shorter courses are as effective and more convenient; for cure, fractionation protects late tissues.