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Correlation between Initial and Long-Term Responses of Spontaneous Pet Animal Tumors to Heat and Radiation or Radiation Alone¹

Division of Radiation Oncology [M. W. D.], Cancer Center [D. A. S., J. L. P.], and Division of Animal Resources [S. W., D. D.], University of Arizona Health Sciences Center, Tucson, Arizona 85724

Most early-phase testing of new therapeutic modalities involves analysis of initial tumor response as opposed to estimation of long-term response. In this study, the validity of initial response rates to predict long-term responses was examined for tumors treated with radiotherapy alone compared with heat combined with radiotherapy.

A total of 130 pet animals with either squamous cell carcinomas, melanomas, fibrosarcomas, mammary adenocarcinomas, or mast cell sarcomas were randomized to receive either radiation alone (XRT) or heat + radiation ( + XRT). Responses to treatment were evaluated by response rates and response duration.

The complete response (CR) rates were consistently higher for + XRT than for XRT across different histology groups. The combined therapy led to prolonged tumor response in all histological subgroups except melanomas, which had a longer response duration when treated with XRT alone (p = 0.043). This was in spite of a relatively high CR rate in that group (100% versus 12.5% for + XRT and XRT, respectively). In contrast, while no significant improvement in CR rate was observed for dermal squamous cell carcinomas treated with + XRT (XRT = 52.9%; + XRT = 68.8%), a significant improvement in response duration was noted (p = 0.002). These are two examples where CR rate did not predict long-term response.

When all histological subgroups were combined (except melanomas), the CR rate was higher (p < 0.001), and response duration was prolonged (p = 0.031) for + XRT compared to XRT alone.

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

² To whom requests for reprints should be addressed.

Received 11/24/82. Accepted 8/26/83.