Clinical Oncology and Radiobiology
Clinical oncology and radiobiology tests how tumour biology, staging, treatment intent, and fractionation shape radiotherapy. Expect the linear-quadratic model, the five Rs, acute versus late effects, and common organ tolerances.
Oncology framework
Staging describes anatomical extent, commonly TNM; performance status and comorbidity affect radical versus palliative intent. Radical treatment aims for long-term control with acceptable normal-tissue toxicity; palliative treatment prioritizes symptom relief and quality of life. Combined chemotherapy and radiotherapy may be concurrent or sequential; concurrent treatment increases both tumour kill and normal-tissue reaction. Common emergencies include spinal cord compression, superior vena cava obstruction, and tumour lysis risk, where urgent oncology input matters.
Radiobiology
The linear-quadratic model describes survival as exp(-αD-βD²). The α/β ratio is the dose at which linear and quadratic contributions are equal. Early-responding tumours and acute effects often have α/β around 10 Gy; late effects often around 3 Gy. Fractionation exploits repair: smaller fractions spare late-responding tissues more than tumour. The five Rs are repair, redistribution, repopulation, reoxygenation, and radiosensitivity. Accelerated repopulation during fractionated treatment can reduce effectiveness, supporting altered fractionation in some head and neck cancers.
Worked example
Compare 30 fractions of 2 Gy with 20 fractions of 3 Gy, both giving 60 Gy. BED=nd(1+d/(α/β)). For tumour α/β=10: 30×2×(1+2/10)=72 Gy10; 20×3×(1+3/10)=78 Gy10. For late tissue α/β=3: 30×2×(1+2/3)=100 Gy3; 20×3×(1+3/3)=120 Gy3. The 3 Gy schedule gives more tumour BED but a much larger late-tissue BED, so the therapeutic ratio may worsen. BED values are comparable only within the same α/β.
Traps
Do not compare Gy10 with Gy3. A higher total dose is not automatically more effective if fraction size and overall time differ. Acute reactions are not simply early and harmless; severe mucositis can interrupt treatment. Reoxygenation improves kill after fractions, while repopulation can reduce it. Organ tolerance depends on volume, dose per fraction, and comorbidity, not just a single maximum dose.