[Clinical application of altered fractionation theory]

Altered fractionation schedules are based on two different concepts of radiobiology. One concept is that the radiation repair capability of cells in late responding tissues is higher than that of cells in acute responding tissues which include tumor tissues. Hyperfractionation utilizes this concept. The other concept is that accelerated repopulation of tumor cells occurs in a later period of radiation therapy. In order to overcome repopulation of tumor cells, accelerated hyperfractionation is proposed. These two concepts are independent and some fractionation methods include both concepts. Clinical results of altered fractionation schedules of radiation therapy could be predicted with a biological model (the linear quadratic model theory)for fractionated radiation. When this biological model is applied to tumors in which the tumor cell repopulation during radiotherapy period is negligible, the correction for tumor proliferation is required. Since the calculation of the biological effect dose with this model uses several assumptions, we should consider the biological effect dose as a crude approximation. Especially, in case of concomitant chemotherapy and altered fractionation, it is difficult to predict its results with a simple radiobiology model. The randomized trial is required to examine the significance of chemoradiotherapy using altered fractionation.