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Thermal Dose Is Related to Duration of Local Control in Canine Sarcomas Treated with Thermoradiotherapy

Authors' Affiliations: ¹ College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina; ² College of Veterinary Medicine, Colorado State University, Fort Collins, Colorado; Departments of ³ Biostatistics and Bioinformatics, ⁴ Radiation Oncology, and ⁵ Medicine, Duke University Medical Center, Durham, North Carolina; and ⁶ Department of Biostatistics and Applied Mathematics, M.D. Anderson Cancer Center, The University of Texas, Houston, Texas

Requests for reprints: Donald E. Thrall, Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606. Phone: 919-513-6292; Fax: 919-513-6578; E-mail: don_thrall{at}ncsu.edu.

Purpose: To test that prospective delivery of higher thermal dose is associated with longer tumor control duration.

Experimental Design: 122 dogs with a heatable soft tissue sarcoma were randomized to receive a low (2-5 CEM43°CT90) or high (20-50 CEM43°CT90) thermal dose in combination with radiotherapy. Most dogs (90%) received four to six hyperthermia treatments over 5 weeks.

Results: In the primary analysis, median (95% confidence interval) duration of local control in the low-dose group was 1.2 (0.7-2.1) years versus 1.9 (1.4-3.2) years in the high-dose group (log-rank P = 0.28). The probability (95% confidence interval) of tumor control at 1 year in the low-dose versus high-dose groups was 0.57 (0.43-0.70) versus 0.74 (0.62-0.86), respectively. Using multivariable procedure, thermal dose group (P = 0.023), total duration of heating (P = 0.008), tumor volume (P = 0.041), and tumor grade (P = 0.027) were significantly related to duration of local tumor control. When correcting for volume, grade, and duration of heating, dogs in the low-dose group were 2.3 times as likely to experience local failure.

Conclusions: Thermal dose is directly related to local control duration in irradiated canine sarcomas. Longer heating being associated with shorter local tumor control was unexpected. However, the effect of thermal dose on tumor control was stronger than for heating duration. The heating duration effect is possibly mediated through deleterious effects on tumor oxygenation. These results are the first to show the value of prospectively controlled thermal dose in achieving local tumor control with thermoradiotherapy, and they establish a paradigm for prescribing thermoradiotherapy and writing a thermal prescription.

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