Hyperthermia Promotes the Incidence of Tumours Following X-irradiation of the Rat Cervical Cord Region

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Abstract
The cervical region of the rat, including the spinal cord (cervical 5-thoracic 2) was irradiated with single doses of 15–32 Gy 250 kV X-rays. Hyperthermia, at temperatures of 42-, 43- and 44 ± 0·1°C for 30 min was applied to the cervical vertebral column and immediate adjacent tissues for 5–10 min or 7 h after X-irradiation. Over a period of 18–21 months, animals were followed up to monitor neurological complications occurring as a result of damage to the spinal cord (Sminia et al. 1991). We also noted the development of neoplasms either inside or outside the cervical region. The data on tumour incidence were analysed retrospectively using the actuarial method. Although hyperthermia alone was not carcinogenic, it led to a significant increase of radiation-induced tumours. This increase of radiation carcinogenesis was observed both with hyperthermia applied 5–10 min after X-rays and with an interval of 7 h between X-rays and heat. Cancer induction was highest after the lower radiation doses (16 Gy) combined with high heat doses (30 min 44°C). The latent period for induction of tumours by X-rays was 472 ± 19 days (mean ± SEM; n = 24). Latency was significantly shortened by hyperthermia to 404 ± 34 days (n = 22) if applied 5–10 min after X-rays and to 348 ± 6 days (n = 33) with an interval of 7 h. Histology revealed that 86% (38/44) of the examined tumours found inside the volume treated with hyperthermia and irradiation were sarcomas. The percentage of animals with a tumour outside the treated volume was almost the same for all treatment groups. Most of these tumours were of the mammary gland type.