(S.S., A.S., A.P., M.B., M.J., J.K. W.)C3H/HeA mice with high incidence of spontaneous breast cancer and Balb1c mice treated with 3,4-benzopyrene (BP) (by painting of the skin resulting in the development of skin cancer) were irradiated with 2,450-MHz microwaves (MW) in an anechoic chamber at 5 or 15 mW/cm2 (2 h daily, 6 sessions per week). C3WHeA mice were irradiated from the 6th week of life, up to the 12th month of life. Balb1c mice treated with BP were irradiated either prior to (over 1 or 3 months) or simultaneously with BP treatment (over 5 months). The appearance of palpable tumors in C,H/HeA mice and of skin cancer in BP-treated Balbic mice was checked every 2 weeks for 12 months. Two additional groups of mice were exposed to chronic stress caused by confinement or to sham-irradiation in an anechoic chamber; these served as controls. Irradiation with M W s at either 5 or 15 mW1cmZ for 3 months resulted in a significant lowering of natural antineoplastic resistance (mean number of lung neoplastic colonies was 2.8 f 1.6 (SD) in controls, 6.1 t 1.8 in mice exposed at 5 mW/cmz and 10.8 2 2.1 in those irradiated at 15 mW1 cm2) and acceleration of development of BP-induced skin cancer (285 days in controls, 230 days for 5 mW/cm* and 160 days for 15 mW/cm2). Microwave-exposed C&I/HeA mice developed breast tumors earlier than controls (322 days in controls, 261 days for 5 mW/cmz and 219 days for 15 mWicm'). A similar acceleration was observed in the development of BP-induced skin cancer in mice exposed simultaneously to BP and MWs (285 days in controls, 220 day for 5 mWicmZ and 121 days for 15 mW1cm'). The acceleration of cancer development in all tested systems and lowering of natural antineoplastic resistance was similar in mice exposed to MW at 5 mW/cm' or to chronic stress caused by confinement but differed significantly from the data obtained on animals exposed at 15 mWicm2, where local thermal effects ("hot" spots) were possible.

Development and growth of skin cancer may be affected by various physical and chemical factors present in human environment. Of these factors electromagnetic radiation of radio- and microwave spectra are among the most common. In the present study Balb/c mice were exposed to chemical carcinogen, 3,4-benzopyrene, painted on the skin every 2nd day for a total of 6 months, and simultaneously irradiated with athermal (5 mW/cm2) or subthermal (15 mW/cm2) doses of 2,450 MHz microwaves. The other group of animals was preirradiated with microwaves at 10 mW/cm2 power level for 1, 2, or 3 months and then treated with benzopyrene, as above. Control mice were exposed for 6 months to benzopyrene, resulting in the development of baso- or spinocellular skin carcinoma within approximately 9 months, and sham-irradiated with microwaves. The growth of the tumour was assessed according to a self-designed 7-range macroscopic scale, supported by microscopical examinations of skin sections. All protocols of microwave irradiations resulted in a significant acceleration of the development of benzopyrene-induced skin cancer and in shortening of life span of the tumour-bearing hosts. This effect seemed to be dose-dependent since subthermal doses (15 mV/cm2) and longer (3 months) expositions to microwaves were more efficient as compared to athermal doses (5 mW/cm2) and shorter preirradiations. In addition, low-level, long-lasting exposure to microwaves led to a marked suppression of delayed hypersensitivity of mice treated with benzopyrene, as assessed by their reactivity to dinitrofluorbenzene (DNFB). It is suggested that the observed co-carcinogenic effect of microwave radiation may, at least in part, result from the inhibitory action of microwaves on cellular immune reactions of exposed animals.