Hematopoietic neoplasia in C57BL/6 mice exposed to split-dose ionizing radiation and circularly polarized 60 Hz magnetic fields

This study assessed the effect of chronic exposure to a 60 Hz circularly po larized magnetic field on the occurrence of ionizing radiation-induced lymp homa and other hematopoietic neoplasia in mice. Female C57BL/6 mice receive d lifetime exposure to either a magnetic field flux density of 1.42 mT for 18 h/day, or an ambient magnetic field of 0.13 mu T Beginning- on the first day of magnetic field exposure, 1710 mice were treated with one of three l evels of split-dose Cobalt-60 gamma-radiation (cumulative 3.0, 4.0 or 5.1 C y), The remaining 570 mice received sham irradiation treatment, Sections fr om 10 lymphoid tissues were evaluated histopathologically for hematopoietic neoplasia, The primary statistical analysis used the Poly3 method to compa re lymphoma incidences in magnetic field (MF)-exposed and control mice. Sec ondary analyses used the Cox proportional hazards model to analyze incidenc e rates for mortality and development of specific types of neoplasia, The m ortality incidence rate was increased by ionizing radiation treatment, and all neoplasms were observed sooner in irradiated mice. However, the lifetim e incidence of hematopoietic neoplasia was similar in all experimental grou ps, including those that were not exposed to ionizing radiation. Chronic ex posure to MFs did not affect the mortality incidence rates and did not chan ge the relative incidences of hematopoietic neoplasia in mice that received the same ionizing radiation treatment, with the exception of a marginally significant reduced relative risk of 0.97 (P = 0.05) for lymphoblastic lymp homa in mice exposed to a magnetic field and treated with 5.1 Cy, Lymphomas and histiocytic sarcomas were first observed similar to 50 days sooner in mice that were exposed to magnetic fields but not ionizing radiation, altho ugh this comparison was not statistically significant and the incidence of hematopoietic neoplasia in these mice was not different from that of mice i n the 0 T/0 Gy group.