Acceleration of mammary tumorigenesis by exposure of 7,12-dimethylbenz[a]anthracene-treated female rats in a 50-Hz, 100-microT magnetic field: replication study.

In view of the methodological problems of epidemiological studies on associations between residential and occupational exposures to 50/60-Hz magnetic fields (MF) and increased incidence of cancers, laboratory studies are necessary to determine if 50/60-Hz MF can affect cancer development or growth. Recently, it was reported that alternating (50-Hz) MF of low flux density (100 microT) increase tumor growth and progression in a model of breast cancer in female rats in which mammary tumors were induced by the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). The objective of the present study was to determine if a replicate experiment carried out in the same laboratory under the same experimental conditions yields a significant increase in tumor development and growth of similar magnitude. For the MF experiment, a group of 99 female Sprague-Dawley rats was exposed to a homogeneous horizontally polarized MF for 24 h/d (minus time for weighing, tumor palpation, cage cleaning, cage rotation), 7 d/wk; another group of 99 rats was sham exposed. DMBA was administered intragastrically at a dose of 5 mg/rat at the first day of exposure and at weekly intervals thereafter up to a total dose of 20 mg/rat. Duration of MF or sham exposure was 91 d. In both MF-exposed and sham-exposed rats, the first tumors could be recorded 6 wk after the initial DMBA application. At 9 wk after DMBA application, the group of MF-exposed rats exhibited significantly more animals with tumors than the sham-exposed group. This significant difference in the rate of tumor development was observed throughout the subsequent period of exposure. After autopsy, the incidence of macroscopically visible mammary tumors was 62% in controls, but 83% in MF-exposed rats, with the 35% difference between groups being statistically significant. Data substantiate that long-term exposure of DMBA-treated female Sprague-Dawley rats in an alternating MF of low flux density promotes the development and growth of mammary tumors, thus indicating that MF exposure exerts tumor-promoting and/or copromoting effects. Furthermore, the data show that the effects of MF exposure in the DMBA breast cancer model are reproducible if the same experiment is repeated in the same laboratory.