Accelerated versus conventional fractionation. The degree of incomplete repair in human skin with a four-hour-fraction interval studied after postmastectomy irradiation.

A previously presented clinical assay with postoperative irradiation to bilateral parasternal fields in patients with breast cancer was used for a comparison of acute and late reactions in human skin after accelerated and conventional fractionation. Two and 3 fractions per day at 4-hour intervals were compared with one fraction per day. Dose fractions of about 2 Gy were used. Twenty-five fractions were given in 2.5, 1.5 and 5 weeks respectively. The acute reactions were similar regardless if 1, 2 or 3 fractions per day were given, i.e. equal total doses were isoeffective. The repair of intracellular damage was apparently completed within 4 hours. However, this might not be true due to a differential influence of proliferation and redistribution on the effect of different types of fractionation, which makes it difficult to interpret the result and estimate the degree of intracellular repair. The time to the acute peak reaction was shortened by 6 to 7 days with accelerated compared to conventional fractionation, explained by the differences in the dose delivery rates. Consequently, the onset of a compensatory proliferation is earlier after accelerated fractionation. Late reactions were more pronounced after accelerated than after conventional fractionation and 1 X 2.0 Gy/day was found to be equivalent to 2 X 1.80 Gy/day and 3 X 1.65 Gy/day at 4-hour intervals with an equal fraction number for all 3 schedules. Assuming that proliferation is negligible for late responding tissues, we interpret this finding as an expression of the degree of reduced intracellular repair. Finally, we would like to point out that the iso-effect dose relationships between acute and late reactions for accelerated versus conventional fractionation might vary, above all with the cell proliferation kinetics of acutely reacting tissue.