Radiation Oncology and Locoregional Treatment
What this domain tests
This domain examines radiobiology, fractionation arithmetic, technique selection and the acute versus late toxicity profile of radiotherapy. You are expected to reason with the linear-quadratic model rather than memorise schedules.
Core rules
- The four R's of fractionation: repair of sublethal damage, redistribution through the cell cycle, repopulation of tumour clonogens, and reoxygenation.
- Linear-quadratic model: BED = nd × (1 + d/(α/β)), where n is the number of fractions and d is dose per fraction. Tumours and early-responding tissues have α/β around 10 Gy; late-responding tissues (spinal cord, lung, kidney) around 3 Gy.
- Conventional fractionation is 1.8–2 Gy per fraction to 60–70 Gy for curative epithelial tumours. Hypofractionation exploits the lower α/β of prostate cancer and breast cancer; SBRT delivers ablative doses in 1–8 fractions with steep dose gradients and mandatory image guidance.
- Tolerance examples (QUANTEC, conventionally fractionated): spinal cord maximum about 45–50 Gy; whole-lung mean dose kept low; parotid mean dose below 26 Gy to reduce xerostomia.
- Palliative schedules: 8 Gy in one fraction, 20 Gy in 5 fractions and 30 Gy in 10 fractions give comparable pain relief in uncomplicated bone metastases, but single fractions have higher retreatment rates.
Worked example
Compare 30 Gy in 10 fractions with 8 Gy in a single fraction for bone pain, using α/β = 10 Gy. First schedule: 10 × 3 × (1 + 0.3) = 39 Gy₁₀. Second: 8 × 1 × (1 + 0.8) = 14.4 Gy₁₀. The larger schedule is almost three times the biological dose, yet trials show similar pain response. Reasoning: tumour kill is not the only mechanism in bone pain, and single-fraction treatment is more convenient for patients with limited prognosis.
Traps
Applying α/β = 10 when predicting late toxicity; forgetting that BED comparisons are invalid across different α/β values; assuming that concurrent chemoradiotherapy is safe without checking the spinal cord constraint.