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Lymphoid Tumor Incidence in Mice Treated with Estrogen and X-Radiation^*

( Department of Radiology, Stanford University School of Medicine, San Francisco, Calif.)

Studies on the combined action of x-radiation and estrogen on lymphoid tumor induction in C57BL and BALB mice are reported. Estrogen markedly augmented the lymphomogenic action of x-radiation in both male and female C57BL mice; this strain is refractory to estrogen alone. Castrated male C57BL mice treated with estrogen had an elevated tumor incidence similar to that of intact animals and significantly greater than the increase noted after castration alone.

Thigh shielding during x-radiation has previously been shown to protect C57BL mice against lymphoma induction by x-rays. The simultaneous administration of estrogen was shown to largely overcome the inhibitory effect of thigh shielding. However, shielding the region of the thymus during x-radiation suppressed lymphoma development, whether or not estrogen was administered.

X-radiation of BALB mice resulted in a relatively low lymphoma incidence and a prolonged latent period, as compared with C57BL mice. Only two lymphomas developed during an observation period of 1 year among 36 BALB mice treated with estrogen alone. Synergism between estrogen and x-radiation, which has previously been reported in this and other strains, could not be demonstrated in this subline of BALB mice.

^* These studies were supported by grants-in-aid from the National Cancer Institute, U.S. Public Health Service, and the Jane Coffin Childs Memorial Fund for Medical Research.

Received 5/22/56.