The influence of fraction size in radiotherapy on the late normal tissue reaction--I: Comparison of the effects of daily and once-a-week fractionation on human skin.

A comparison of the normal tissue reactions to daily and once-a-week fractionation was performed on postoperatively irradiated bilateral parasternal fields in patients with breast cancer. Both schedules resulted in identical acute skin erythema and pigmentation. However, the development of telangiectasia was significantly more pronounced after once-a-week fractionation. These findings imply that the Cumulative Radiation Effect (CRE) formula has to be modified. The slope of the iso-effect dose versus number of fractions was 0.24 for acute skin reactions and estimated to be about 0.34 for late skin reactions at a dose level equivalent to 46 Gy in 2 Gy fractions with conventional fractionation. The beta/alpha ratio was 0.12 Gy-1 for acute and about 0.31 Gy-1 for late effects, for 200 kV X rays.