Radiobiology of High Dose Fractions

Advances in the technology of radiotherapy delivery have resulted in deliberate radiation fluence and dose displacement away from designated normal tissues, and with improved conformity of tumour dose. This applies to normal tissues outside the planning target volume (PTV) in most es. The prospects for hypofractionation improve in these circumstances provided that loss of function of the normal tissue included in the PTV is not considered harmful or deleterious to the subsequent health and well-being of the patient. The radiobiology of large fractions is considered in the context of the linear quadratic (LQ) model of radiation effect and the concept of the biological effective dose (BED). One feature of the model is that it might overestimate high fractional dose effects especially in tumours or tissues which have low α/β ratios. For normal tissues, this is probably advantageous since the model provides a ‘worst e scenario’, and protects against overdosage. Substantial benefits in the therapeutic ratio with increasing fractionation only apply where there is a marked difference between the α/β ratios of the tumour and relevant normal tissues. Thus slow growing tumours with low α/β ratios are preferred candidates for hypofractionation. Where high dose fractions are employed it is vital to ensure that the prescribed dose is not exceeded in relevant normal tissue where overdosage can be harmful. Some worked examples are given to illustrate these principles, using BED calculations, with examples of how to include straightening out of the dose response curve.