Hypofractionation in radiotherapy

Major changes in cancer radiotherapy have followed a greater understanding of the biological effects of radiation on tumours and normal tissues. Clinical radiotherapy is today a solid body of knowledge with well defined scientific foundations. Key concepts in current radiobiology include lethal and sublethal injuries, dose-effect coefficients, alpha/beta ratios, acute and late response, biologically equivalent dose, fraction dose, irradiation time and tumour regeneration between others. Effects of irradiation time and dose per fraction on tumours versus normal tissues are of special importance. Dose per fraction must be considered for analysis of effects in normal late-responding tissues. In contrast, both dose per fraction and irradiation time influence the response to radiation of malignant tumours and acute-responding tissues. Finally, the ability to quantify relationships between radiation dose and biological effect has been of particular value in the development of radiotherapy. This is illustrated by the growing use of high doses per fraction for the treatment of some cancers.