Hypofractionated irradiation for non-small cell lung cancer.

Large radiation fractions are an effective way of killing tumour cells but have generally been avoided in curative treatment of patients because of concerns of a disproportionate increase in late normal tissue toxicity. Radiobiological modelling of the effect of radiation on lung tumours and late-reacting normal tissues, which are more sensitive to large radiation fractions, has been undertaken. The biological effect of radiation on tumours is increased as the overall treatment time is shortened but this is not true for late-reacting normal tissue. Sample data are shown in which the relative increases in radiation effect on the tumour and late-reacting normal tissues are similar after hypofractionation. A favourable therapeutic ratio can be achieved because the bulk of normal tissue will receive a lower dose of radiation at a lower dose per fraction than the tumour, especially with current techniques where the volume of normal tissue irradiated can be sharply reduced. The clinical evidence confirms that lung toxicity is volume-dependent. It is the small Stage I and II tumours which are most likely to benefit from hypofractionated regimens, as the volumes to be treated are smaller and they have a lower incidence of distant metastases. Patients with Stage III tumours with favourable prognostic factors are nowadays treated with combined chemotherapy and radiotherapy and so for this group more conservative hypofractionation regimens are being explored. However, more advanced tumours may be treated with hypofractionation to lower total doses to achieve palliation and a modest degree of survival benefit.