Development of osteosarcoma in rats after irradiation.

The rat femur diaphysis was exposed to local continuous irradiation with various constant dose-rates for different periods of time and examined after different observation times. The incidence of osteogenic sarcomas per cm2 appeared to be comparable to that reported for animals exposed to internal irradiation by bone-seeking isotopes which involves systemic effects. The site of origin in 5 rats was found to be in or near the endosteum. Malignant proliferation started after at least 180 days of irradiation. Tumour development was dependent upon continued irradiation of an endosteum which contained a thin layer of fibroblasts and a few apparently inactive osteoblasts and osteoclasts. No increased cellular activity and no indication of new bone, cartilage or fibrosis were evident before the tumours began to develop.