ACCELERATED PAPER: Intermittent 50 Hz magnetic field and skin tumour promotion in SENCAR mice

A number of epidemiological studies have indicated associationbetween exposure to extremely low frequency electromagneticfields and a variety of cancers, including leukaemia and braintumours among residentially exposed children and among occupationallyexposed adults. In order to test if intermittent magnetic fields(MF) act as a tumour promoter, a long-term skin carcinogenicitystudy of 50 Hz sinusoidal MF with flux densities of 50 µTand 0.5 mT, continuous as well as with an intermittence of 15s on/off, was performed. Female SENCAR mice were divided intoeight groups of 50 animals in each and treated according toan initiation- promotion scheme. 7,12-dimethylbenza] anthracene(DMBA) in acetone was applied to the dorsal skin at a subcarcinogenicdose, as an initiator and exposure to MF was performed for 19–21h/day during 104 weeks starting 1 week after the initiator treatment.The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA)was used as a positive control for skin tumour promoting activity.Two animals from each group were assigned for skin hyperplasiaanalysis at 2, 6, 12,18 and 21 months. The animals were observeddaily. The appearance of skin lesions and neoplasms were carefullyfollowed and histopathological diagnosis was made for all neoplasmspresent at death. The experiment was terminated after 105 weeks.DMBA-treatment alone yielded altogether two skin tumours intwo tumour-bearing animals and the animals exposed to acetonealone had one skin tumour. The animals exposed to continuousfields showed no skin tumour. Five animals exposed to 0.5 µTon/off had a total of 13 skin tumours and in the group exposedto 50 µT on/off four animals had a total of four skintumours. The on/off exposed groups differed significantly fromthe continuously exposed groups (P = 0.014) but the differencebetween the on/off exposure groups and the DMBA group was notstatistically significant when tumour-bearing animals and cumulatedskin tumours were compared. There was a statistically significantdose trend (P = 0.045) with flux density and Tesla-h for intermittentMF exposure for cumulated skin tumours per tumour-bearing animals.The epithelial thickness of DMBA + MF-treated animals was ofthe same magnitude as for DMBA-treated animals indicating that,in the case of a promoting effect being present, another mechanismthan one involving sustained hyperplasia may be involved.