In external beam radiotherapy for head and neck cancer, the total prescribed dose is conventionally delivered in many small daily fractions spread over several weeks rather than as one single large fraction. The principal radiobiological reason for fractionation is:

• A It increases the relative biological effectiveness of photons compared with protons
• B Single large doses selectively spare hypoxic tumour cells at the centre of the mass
• C Normal tissues repair sublethal DNA damage between fractions better than tumours, widening the therapeutic window ✓
• D Fractionation shifts tumour cells into the radioresistant S phase of the cell cycle

Explanation

Fractionation exploits the four Rs of radiobiology: repair of sublethal damage is more efficient in normal tissues than in tumours, allowing a higher total tumoricidal dose while sparing late-responding normal tissue. Redistribution into sensitive phases and reoxygenation of previously hypoxic cells further favour the tumour, which makes option B wrong since hypoxic cells are actually spared by single doses. Relative biological effectiveness is a property of the radiation type, not the schedule.

Reference: DeVita, Hellman, and Rosenberg's Cancer: Principles and Practice of Oncology, 12th ed.

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Written and medically reviewed by the StethoPrep medical team.