Linear relationship between flux density and tumor co-promoting effect of prolonged magnetic field exposure in a breast cancer model.

Previous epidemiologic studies have suggested that exposure to 50- or 60-Hz (power-frequency) magnetic fields in occupational or residential environments may increase the risk of certain cancers, including breast cancer. However, in view of the methodological problems of epidemiological studies on associations between magnetic field exposures and increased incidence of cancers, laboratory studies are necessary to determine if 50/60-Hz magnetic fields are cancer promoters or can progress cancers. The objective of the present study was to characterize the relation, if any, between dose (i.e. flux density) of 50-Hz magnetic field exposure and tumor growth in a model of breast cancer in female rats. Mammary tumors were induced by the chemical carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA). The dosing protocol of DMBA chosen for the magnetic field experiments induced mammary tumors in about 50% of sham-exposed control animals within 3 months after application. Per flux density, a group of 36-99 rats was exposed to a magnetic field of 50-Hz for 24 h/day 7 days/week; another group of 36-99 rats was sham-exposed under the same environmental conditions as the MF-exposed rats. The exposure chambers were identical for MF-exposed and sham-exposed animals. DMBA was administered orally at a dose of 5 mg per rat at the first day of exposure and at weekly intervals thereafter up to a total dose of 20 g per rat. Duration of MF- or sham-exposure was 91 days. At the end of the exposure period, the animals were sacrificed for examination of the number of mammary tumors. Four flux densities were studied in a total of 666 rats (including sham-exposed controls): 0.3-1 muT, 10 muT, 50 muT, and 100 muT. At autopsy, i.e. at the end of the 13 weeks period of MF-exposure, incidence of macroscopically visible mammary tumors was significantly enhanced in the experiment with 50 muT (25.5% above control) and 100 muT (50% above control). No increase in incidence of mammary tumors was seen in the experiment with 0.3-1 muT, while a 10% (non-significant) increase was determined in the experiment with 10 muT. Linear regression analysis of the data from the four experiments indicated a highly significant linear relation between flux density and increase in tumor incidence at time of autopsy. The correlation coefficient was 0.9944 (P < 0.01). The data demonstrate that long-term exposure of DMBA-treated female rats increases the growth of mammary tumors in a highly dose-related fashion.(ABSTRACT TRUNCATED AT 400 WORDS)