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Importance of Minimum Tumor Temperature in Determining Early and Long-Term Responses of Spontaneous Canine and Feline Tumors to Heat and Radiation¹

Division of Radiation Oncology [M. W. D., W. G. C.] and Cancer Center [D. A. S.], Arizona Health Sciences Center, Tucson, Arizona 85724, and Department of Medicine, Wayne State University, Detroit, Michigan 48202 [S. S.]

A total of 130 dogs and cats with squamous cell carcinomas, melanomas, fibrosarcomas, mammary adenocarcinomas, or mast cell sarcomas were randomized to receive radiation (XRT) or heat plus XRT. Time-temperature data for each monitored tumor location were converted to degree-minutes or equivalent min at 43° (Eq43).

Response rates and durations of response were compared for subgroups of histology, volume, site, and heat treatment method. Thermal gradients existed in all heated tumors. The influence of these gradients on tumor response was examined by correlation of response with degree-minutes and Eq43 minima, maxima, averages, and ranges. A pattern emerged from these analyses linking dose minima, maxima, and ranges with prognostic subgroups as classified by volume, site, or treatment method.

The data indicated that the coolest part of the tumor governed the biological response to combined heat + XRT. Tumors which received a minimum of 35 Eq43 had significantly longer durations of response than did those receiving XRT alone or <3 Eq43 (p 0.006 and 0.014, respectively; log-rank test). Furthermore, broad temperature ranges were associated with power-limiting "hot spots" and invariably led to underheating in other areas of tumor. Multivariate analysis found minimum Eq43 on the first treatment to be the best predictor of long-term response (p < 0.05). Other biological covariates of site, volume, and histology contributed strength to the model, which was independent of Eq43 (p < 0.05).

¹ This work was supported by Grants CA17343 and CA23074 from the National Cancer Institute, NIH. Presented at the 24th Annual Meeting of the American Society of Therapeutic Radiologists, October 25 to 29, 1982, Orlando, Fla. (10).

² To whom requests for reprints should be addressed.

Received 12/20/82. Accepted 9/14/83.

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