Exposure of Sprague-Dawley rats to a 50-Hertz, 100-mu Tesla magnetic fieldfor 27 weeks facilitates mammary tumorigenesis in the 7,12-dimethylbenz[a]anthracene model of breast cancer

We have shown previously (W. Loscher et al., Cancer Lett., 71: 75-81, 1993; M, Mevissen st at, Carcinogenesis (Lond,), 17: 903-910, 1996) that 50-Hz m agnetic fields (MFs) of low [50 or 100 mu Tesla (T)] flux density enhance m ammary gland tumor development and growth in the 7,12-dimethylbenz[a]anthra cene (DMBA) model of breast cancer in female Sprague Dawley rats. In these previous experiments, groups of rats were given 20 mg of DMBA (four weekly gavage doses of 5 mg each) and were MF- or sham-exposed for 13 weeks. The o bjective of the present study was to examine whether the use of a lower dos e of DMBA (10 instead of 20 mg per rat), MF exposure of the rats before DMB A injection, and the increase of the MP exposure period after DMBA applicat ion to 26 weeks enhance the effect of MF on tumor development and growth. A group 99 rats was exposed to a homogeneous, horizontally polarized 100-mu T MF of 50;Hz for 24 h/day for 7 days/week; another group of 99 rats was sh am-exposed under the same environmental conditions as the MF-exposed rats. The exposure chambers were identical for MF-exposed and sham-exposed animal s. The age of the rats was 45-49 days at the onset of exposure; duration of MF or sham exposure was 27 weeks. DMBA was administered p.o. at a dose of 10 mg/rat after 1 week of MF or sham exposure. The animals were palpated on ce weekly from week 6 onwards to assess the development of mammary tumors. At the end of the exposure period, the animals were killed for the determin ation of number and volume and histological verification of mammary tumors. All of the recordings were done in a blinded fashion; ie., the investigato rs were not aware which animals were MF- or sham-exposed, Mammary tumor dev elopment and growth was significantly enhanced by MF exposure, the most mar ked effect on tumor incidence (190% above sham control) being observed 13 w eeks after DMBA administration. At the time of necropsy, i.e., 26 weeks aft er DMBA administration, the incidence of histologically verified mammary tu mors was 50.5% in controls and 64.7% in MF-exposed rats, the difference bei ng statistically significant. More marked intergroup differences were recor ded when tumor incidence was separately evaluated for each of the six mamma ry complexes, the most pronounced MF effect on tumor incidence being seen i n the cranial thoracic complex. The data substantiate that, at least under the experimental conditions used in our laboratory, 50-Hz, 100-mu T MF expo sure significantly facilitates the development and growth of mammary tumors in the DMBA rat model of breast cancer.