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Our previous study on early- and late-appearing murine foot reactions following combined hyperthermia and irradiation suggested that hyperthermia may not inhibit repair of sublethal and/or potentially lethal radiation damage for the late-appearing reaction. A series of split dose experiments was performed by using C3Hf/Sed mouse foot reaction, which showed both early and late responses in the same tissue. Hyperthermia given in a 43.5 degrees C water bath for 45 min inhibited radiation damage repair in the early-appearing reaction, but was not able to modify repair in the late-appearing reaction. The failure of thermal inhibition of radiation damage repair in the late-appearing reaction was observed regardless of the sequence of heat and irradiation. Complete repair appeared to require approximately 9 hr. Although it is unknown whether radiation damage repair can be completely inhibited in all late-responding normal tissues, the reduction in thermal repair inhibition in some late-responding tissues may be an advantage of "heat prior to irradiation" treatment over radiation alone.

The effect of twice-a-day irradiation on the foot reaction was studied in C3Hf/Sed mice. A numerical scoring system was used to evaluate the foot reaction that became visible shortly after irradiation. Peak reaction occurred on the 21st to 23rd day and reappeared after approximately the 200th postirradiation day. These reactions are called early- and late-appearing reactions, respectively. Animals studied for early-appearing reactions were continuously scored for more than 600 days after irradiation. The late-appearing reaction increased continuously or remained constant, but never decreased during the course of the experiments. Animal feet were irradiated with a fraction size from 1.0 to 5.0 Gy twice-a-day daily with a treatment interval of 6 and 18 hours, and varying numbers of fractions were given. A top-up dose of 20 Gy was given as the last dose. The late-appearing foot reaction following a fraction size of 1.5 Gy or greater increased with increasing total dose. However, the increase in the late reaction score with increasing total dose was trivial following multiple doses with a fraction size of 1.0 Gy, suggesting a possible repopulation during prolonged irradiation or that the observed reaction was a consequential late effect. The Fe-plot based on doses which induced fibrosis in one-half of the irradiated animals, showed an alpha/beta ratio of 6.57 +/- 0.62 Gy, and the presence of a breaking point at approximately 1.5 Gy. The alpha/beta ratios calculated by Joiner's method and Thames direct analysis were identical. The slope of the Fe-plot below 1.5 Gy was significantly steeper than that above 1.5 Gy.

We studied the reaction of the mouse foot after combined X-irradiation and heat treatment. Acute reactions after heat differ from those after irradiation, however, after healing of the lesions, the same symptoms of deformity of the mouse foot remain. Prior heat treatment, 30 min at 43 degrees C, of the foot led to thermotolerance and this thermotolerance resulted in resistance to combined irradiation-heat treatments and hence to a decreased thermal enhancement of radiation effects. Resistance could be observed up to 168 h after prior heat treatment. The development of resistance to combined treatment at higher irradiation dose (15 or 20 Gy) and less severe heating was slower than at lower irradiation dose (10 Gy) and more severe heating. Thermal enhancement was confirmed to be dependent on the sequence of, and the interval between irradiation and heat treatment. When the mouse foot was made thermotolerant by prior heat treatment, thermal enhancement was always reduced, regardless of the sequence, when the combined heat and radiation treatments were given with an interval of less than 12 h. Thermotolerance led to an apparent decrease in the effective temperature employed in a combined treatment equivalent to approximately 1.0 degrees C, at temperatures above 43 degrees C in a 1 h heat treatment.