Gamma-ray induction of malignant tumors in rats.

Sources of protracted gamma radiation adjacent to bone were found to induceosteogenic sarcoma in half of the exposed males. Osteogenic sarcomas appeared in 3 males remote from the radioactive sources and may not have been radiation induced. Some tumors were small enough so that the site of origin could be determined accurately: some arose from endosteum, others from periosteum. Although portions of the skeleton in the female rats incidentally received doses of over 20,000 rads, no osteogenic sarcomas developed in them. Parauterine sources induced adenocarcinoma of the endometrium in 14 of 32 rats (44%). The incidence of carcinoma of the ovary (12%) was lower than expected. Carcinomas of the breast were frequent, but often distant from the source. Increased total dose, which was usually related to length of time of exposure, produced an increased incidence of cancer and a shortening of the latent period. However, extremely high doses (over a million rads to bone and hundreds of thousands to the uterus) at times failed to induce cancers.