A HISTOPATHOLOGICAL STUDY ON ALTERATIONS IN DMBA-INDUCED MAMMARY CARCINOGENESIS IN RATS WITH 50-HZ, 100-MU-T MAGNETIC-FIELD EXPOSURE

Several epidemiological studies have indicated that residential or occ upational exposure to 50 or 60 Hz magnetic fields (MF) may increase th e risk of breast cancer, possibly by suppression of pineal production of the oncostatic hormone melatonin, In view of the methodological pro blems of epidemiological studies on MF exposure and cancer risk, labor atory studies are needed to determine whether 50/60 Hz exposure can in itiate, promote or copromote mammary cancer. In the present study, 216 female Sprague-Dawley rats were divided into four groups. Two of the groups (with 99 animals each) received oral applications of 7,12-dimet hylbenz[a]anthracene (DMBA) and mere either sham-exposed or exposed in a 50 Hz, 100 mu T MF for 24 h/day 7 days/week for a period of 91 days . The other two groups (nine animals each) were either sham-exposed or MF-exposed without DMBA treatment. The exposure chambers and all othe r environmental factors were identical for MF-exposed and sham-exposed animals, At the end of the 3 month period of MF exposure, all rats we re used for histopathological diagnosis of lesions, At the time of nec ropsy, significantly more MF-exposed DMBA-treated rats exhibited macro scopically visible mammarS tumours than DMBA-treated controls. Further more, the size of mammary tumours was significantly larger in MF-expos ed rats. Histopathological examination of the mammary gland showed tha t the number of neoplastic and non-neoplastic lesions did not signific antly differ between groups, indicating that MF exposure had not alter ed the incidence of mammary lesions but had only accelerated tumour gr owth, consistent with a co-promoting effect. In the MF-exposed group, significantly more rats exhibited malignant mammary tumours than in co ntrols, indicating that MF exposure had affected the progression of DM BA-induced lesions. The number of metastases of mammary tumours or of primary lesions in other organs in response to DMBA was not affected b y MF exposure. In rats without DMBA application, no non-neoplastic or neoplastic lesions were determined. The data demonstrate that long-ter m exposure of DMBA-treated female rats promotes the growth and progres sion of mammary tumours, while tumour incidence is not affected, at le ast under the experimental conditions of the present study, The data t hus add to the accumulating evidence that MF exposure exerts tumour co -promoting effects.