Effect of ovariectomy on x-ray carcinogenesis in rats

Tumor induction by fractionated whole-body X-irradiation (400 rad) was studied in spayed Sprague-Dawley rats. Ovariectomy was chosen as an intensifying factor for radiation leukemogenesis. Ovariectomized rats gained more body weight and responded more quickly (but transiently) in the recovery of WBC levels after the last (3rd or 4th) X-irradiation. A total of 26 tumors developed in 21 out of 47 ovariectomized rats, 11 tumors in 6 out of 13 ovariectomized and ovary-grafted rats, and 44 tumors in 25 out of 29 sham-ovariectomized rats during the observation period up to 64 weeks after starting X-irradiation. Eighty per cent of tumors were of mammary gland origin in the latter two groups with intact or grafted ovaries. By contrast, 61.1% of tumors in the spayed rats were derived from the subcutaneous mesenchymal tissue and the hematopoietic tissue. This may imply that some forms of mesenchymal tumors including leukemia are under the suppressive influence of female sex hormones.