Microwave hyperthermia in the treatment of spontaneous canine tumours: an analysis of treatment parameters and tumour response.

Fifty-one spontaneous canine tumours were treated with combined radiation and hyperthermia in a pilot study designed to investigate the feasibility and efficacy of the combined modality treatment in the dog. The tumours varied in site and histological type: 35 were in the oral cavity and included squamous cell carcinoma, melanoma and various 'sarcomas'. All animals received radiation (36-40 Gy in four weekly fractions) and post-irradiation hyperthermia (prescription -44 degrees C for 30 min) on one or two occasions. The microwave hyperthermia system was technically satisfactory in elevating tumour temperature to 44 degrees C or above in 95 per cent of treatments. However, thermal gradients of the order of 3-5 degrees C were frequently measured across the tumour, and rarely did all peripheral points achieve the target of 44 degrees C for 30 min. The overall tumour response rate (CR + PR) was 87.7 per cent with 60.7 per cent of tumours achieving complete regression. Smaller lesions showed a significantly greater response rate (P = 0.004) and those lesions which received two thermal treatments show an increased response (P = 0.0095). Fifty-one per cent of tumours showed significant necrosis following hyperthermia. Normal tissue necrosis was seen in three patients; in two cases this was attributed to hyperthermia. Local tumour control rate and necrosis was not correlated with measured minimum, mean or maximum thermal doses.