Carcinogenesis, Vol. 21, No. 7, 1379-1389,
July 2000
© 2000 Oxford University Press
Carcinogenesis |
Hematopoietic neoplasia in C57BL/6 mice exposed to split-dose ionizing radiation and circularly polarized 60 Hz magnetic fields
1 Department of Medicine and
2 Department of Radiology, University of California at Los Angeles, Los Angeles, CA 90095,
3 Immunotherapy Laboratory, St Vincent's Medical Center, Los Angeles, CA 90057,
4 Department of Biostatistics, University of Michigan, Ann Arbor, MI 48109,
5 Merle Norman Cosmetics, El Segundo, CA,
6 Nuclear Medicine and
7 Geriatric Research, Education and Clinical Center, V.A. Greater Los Angeles Healthcare System at West Los Angeles, Los Angeles, CA 90073 and
8 Scirex Corporation, Horsham, PA 19044, USA
This study assessed the effect of chronic exposure to a 60 Hz circularly polarized magnetic field on the occurrence of ionizing radiation-induced lymphoma and other hematopoietic neoplasia in mice. Female C57BL/6 mice received 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 µT. Beginning on the first day of magnetic field exposure, 1710 mice were treated with one of three levels of split-dose Cobalt-60 
-radiation (cumulative 3.0, 4.0 or 5.1 Gy). The remaining 570 mice received sham irradiation treatment. Sections from 10 lymphoid tissues were evaluated histopathologically for hematopoietic neoplasia. The primary statistical analysis used the Poly3 method to compare lymphoma incidences in magnetic field (MF)-exposed and control mice. Secondary analyses used the Cox proportional hazards model to analyze incidence rates for mortality and development of specific types of neoplasia. The mortality incidence rate was increased by ionizing radiation treatment, and all neoplasms were observed sooner in irradiated mice. However, the lifetime incidence of hematopoietic neoplasia was similar in all experimental groups, including those that were not exposed to ionizing radiation. Chronic exposure to MFs did not affect the mortality incidence rates and did not change 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 lymphoma in mice exposed to a magnetic field and treated with 5.1 Gy. Lymphomas and histiocytic sarcomas were first observed ~50 days sooner in mice that were exposed to magnetic fields but not ionizing radiation, although this comparison was not statistically significant and the incidence of hematopoietic neoplasia in these mice was not different from that of mice in the 0 T/0 Gy group.
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