Improvements in radiotherapy fractionation

In general, tumours and critical normal tissues (those which limit the dose which may safely be delivered in a course of radiotherapy) are associated with different fractionation sensitivities. Thus, reducing the daily dose per fraction will allow a greater total dose to be delivered to the tumour without exceeding the normal tissue tolerance and the therapeutic index may be favourably altered.

Studies have demonstrated fast rates of growth in certain tumours (e.g. squamous cell carcinomas of the head and neck region) such that clonogens have potential doubling times of less than five days. This implies that the time taken to deliver a radical course of conventionally fractionated radiotherapy should not be extended. Indeed, clinical trials have demonstrated benefits with treatment acceleration where a radical course of treatment may be completed in two weeks (with thrice daily fractions), rather than the more usual 4-6 weeks.

Although the potential benefits of acceleration and changed frac-tionation are clear, the impact of their adoption on overall survival is likely to be modest and only on subgroups of patients. Even amongst tumours of common histology, there are wide variations in the radio-biological parameters such as potential doubling time.

There is interest in the development of predictive assays, from which it may be possible to tailor a schedule of radiotherapy to the individual tumour.

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