Effects of magnetic fields on mammary tumor development induced by 7, 12‐dimethylbenz(a)anthracene in rats

Abstract

A series of epidemiological studies have indicated associations between exposure to magnetic fields (MFs) and a variety of cancers, including breast cancer. In order to test the possibility that MF acts as a cancer promoter or copromoter, four separate experiments have been conducted in rats in which the effects of chronic exposure to MFs on the development of mammary tumors induced by 7,12-dimethylbenz(a)anthracene (DMBA) were determined. Female rats were exposed in magnetic coils for 91 days (24 h/day) to either alternating current (AC; 50 Hz)-MF or direct current (DC)-MF. Magnetic flux density of the DC-MF was 15 mT. Two AC-MF exposures used a homogeneous field with a flux density of 30 mT (rms); one used a gradient field with flux density ranging from 0.3-1 microT. DMBA (5 mg) was administered orally at the onset of MF exposure and was repeated thrice at intervals of 1 week. In each experiment, 18-36 animals were exposed in 6 magnetic coils. The same number of rats were used as sham-exposed control. These control animals were treated with DMBA and were placed in dummy coils in the same room as the MF-exposed rats. Furthermore, groups of age-matched rats (reference controls) were treated with DMBA but housed in another room to exclude any MF exposure due to the magnetic stray field from the MF produced by coils. At the end of the exposure or sham-exposure period, tumor number and weight or size of tumors were determined at necropsy. Results were as follows: In sham-exposed animals or reference controls, the tumor incidence varied between 50 and 78% in the 4 experiments. The average number of mammary tumors per tumor-bearing animal varied between 1.6 and 2.9. In none of the experiments did MFs significantly alter tumor incidence, but in one of the experiments with AC-MF exposure at 30 mT, the number of tumors per tumor-bearing animal was significantly increased. Furthermore, exposure to a DC-MF at 15 mT significantly enhanced the tumor weight. Exposure to a gradient AC-MF at 0.3-1 microT exerted no significant effects. These experiments seem to indicate that MFs at high flux densities may act as a promoter or copromoter of breast cancer. However, this interpretation must be considered only a tentative conclusion because of the limitations of this study, particularly the small sample size used for MF exposure and the lack of repetition of data.

Significant Differences in the Effects of Magnetic Field Exposure on 7,12-Dimethylbenz(a)anthracene-Induced Mammary Carcinogenesis in Two Substrains of Sprague-Dawley Rats

“…During MF or sham exposure, rats were palpated once per week for the detection of mammary gland tumors in both experiments. The size of palpable tumors was estimated by a rating scale as described recently (27). Furthermore, the location of each tumor among the six mammary complexes of the rat was recorded.…”

Testing Electromagnetic Fields for Potential Carcinogenic Activity: A Critical Review of Animal Models

“…As shown in Table 4, results of two experiments at the 0.3 to 1.0 pT flux density have been reported (34,40,41). In each experiment, a group of 36 rats was exposed to the gradient ac sine wave 50-Hz field and a group of 36 was exposed to a sham field.…”

“…Most work with this system is from Loscher and his colleagues (34,(40)(41)(42)(43)(44)(45)(46). In all experiments, female Sprague-Dawley rats 6 to 8 weeks of age were exposed by gavage to DMBA (four weekly doses at 5 mg/rat/dose) beginning on the first day of MF exposure.…”

“…In an early experiment, small groups of animals (18 in each group) were exposed either to 15-mT dc or 30-mT 50-Hz fields (40). Two separate experiments were performed using 30-mT 50-Hz fields.…”

“…These frequencies fall in the extremely-low-frequency spectrum, defined as 3 Hz to 3 kHz. A few other studies used either 60 (39,40,43,44,46); the rat liver focus system (59); and the mouse skin system (77,80,82,83). It may be of interest that positive effects reported in both the rat mammary gland and the mouse skin systems suggest that EMF at low flux densities may enhance the latter stages of carcinogenesis.…”