Early and late radiation changes following a second course of irradiation.

The effect of two 10-fraction courses of irradiation separated by intervals varying from 1 to 10 months on both the early skin reaction and late deformity of the leg was studied in mice. It was found that the "memory" of the first course was much greater for the late end point (deformity) than for early skin reactions at all intervals between the two courses. The extent of the late deformity reaction following a second course was predicted fairly well from the Ellis formula for decay of partial tolerance, but it also appeared to be more complex than is evident from the formula. Evaluation of early vs. late reactions showed that the extent of the deformity is not governed entirely by the level of the early skin reaction; rather, it depends critically on the previous radiation history of the tissue.