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 brea st cancer. However, in view of the methodological problems of epidemio logical studies on associations between magnetic field exposures and i ncreased incidence of cancers, laboratory studies are necessary to det ermine if 50/60-Hz magnetic fields are cancer promoters or can progres s cancers. The objective of the present study was to characterize the relation, if any, between dose (i.e. flux density) of 50-Hz magnetic f ield exposure and tumor growth in a model of breast cancer in female r ats. Mammary tumors were induced by the chemical carcinogen 7,12-dimet hylbenz[a]anthracene (DMBA). The dosing protocol of DMBA chosen for th e magnetic field experiments induced mammary tumors in about 50% of sh am-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-expo sed under the same environmental conditions as the MF-exposed rats. Th e exposure chambers were identical for MF-exposed and sham-exposed ani mals. DMBA was administered orally at a dose of 5 mg per rat at the fi rst day of exposure and at weekly intervals thereafter up to a total d ose 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 examin ation of the number of mammary tumors. Pour flux densities were studie d in a total of 666 rats (including sham-exposed controls): 0.3-1 mu T , 10 mu T, 50 mu T, and 100 mu T. At autopsy, i.e. at the end of the 1 3 weeks period of MF-exposure, incidence of macroscopically visible ma mmary tumors was significantly enhanced in the experiment with 50 mu T (25.5% above control) and 100 mu T (50% above control). No increase i n incidence of mammary tumors was seen in the experiment with 0.3-1 mu T, while a 10% (nonsignificant) increase was determined in the experi ment with 10 mu T. Linear regression analysis of the data from the fou r experiments indicated a highly significant linear relation between f lux density and increase in tumor incidence at time of autopsy. The co rrelation coefficient was 0.9944 (P < 0.01). The data demonstrate that long-term exposure of DMBA-treated female rats increases the growth o f mammary tumors in a highly dose-related fashion. The data thus add t o the accumulating evidence that magnetic field exposure exerts tumor- co-promoting effects.