The influence of the overall treatment time in radiotherapy on the acute reaction: comparison of the effects of daily and twice-a-week fractionation on human skin.

A comparison of the acute skin erythema and pigmentation after daily fractions of 2 Gy and twice-a-week fractions of 4 Gy was performed on post-operatively irradiated parasternal fields in patients with breast cancer. The radiation quality used was electron beams with an energy of about 12 MeV. The overall treatment time varied between 4 and 6 weeks. The two schedules were also compared for an overall treatment time of 10 weeks, with a rest period of 3 weeks after 4 weeks of irradiation, followed by another 3 weeks of treatment. With 5 to 6 weeks irradiation, daily fractions resulted in a significantly more pronounced skin erythema (p less than 0.05) than twice-a-week fractions, corresponding to about 10% in terms of absorbed dose. With 4 weeks irradiation and split-course therapy, the acute reactions were identical with both schedules. We interpret these findings as indicating a radiation-induced accelerated repopulation of the basal cells in the epidermis, with a fairly abrupt onset after 4 weeks. The higher effect after more frequent fractions of smaller size would be expected from a sensitizing effect of redistribution. In conclusion, the overall treatment time is of importance for establishment of iso-effect relationships for acutely responding tissues. The implication is that different fractionation parameters used in models to predict equivalent radiation effects of different fractionation schedules also vary significantly with the overall treatment time.