Acute and late damage in mouse tail following hyperthermia and X-irradiation

Tissue injury following X-irradiation of adult mouse tail was assessed by: (a) the severity of the acute skin reaction, which appeared on day 13 and peaked at approximately 20 days after treatment; (b) the occurrence of necrosis by three months; and (c) the incidence of "late' necrosis which developed between 6.5 and 11 months after treatment, in tails whose gross appearance was normal at 3 months. Hyperthermia (44.0 or 44.5 degrees C for 30 min, by hot-water immersion) given 9 days after radiation, i.e. close to the time of appearance of the acute skin reaction, resulted in an increased thermal response and, in addition, an increase in tissue response for all three waves of radiation injury studied. The times of expression of injury remained unaltered and the degrees of thermal enhancement of both early and "late' necrosis were similar to that measured for the acute skin reaction following radiation. All values were significantly lower than the thermal enhancement ratio (TER), measured as a result of a primary interaction between heat and radiation when the two modalities are given in close association. The observed increase in "late' injury could be accounted for by thermal potentiation of the acute radiation response; there was no evidence for a significant exacerbation by a "late' or progressing thermal injury.