Malignant Hemangioendothelioma in an Irradiated Rat

A male hybrid F₁ rat developed a tumor on the lateral side of the right thigh, which had been exposed to a high dose of external radiation (3,500 rads), administered as a single dose by an X-ray machine 16 months previously. The tumor grew rapidly and showed extensive local spread, but there were no metastases at autopsy. The tumor was ulcerated, dark red and hemorrhagic. It was not encapsulated and invaded the surrounding muscles extensively. Microscopically the tumor was composed of anastomozing channels lined by swollen anaplastic endothelial cells that were either heaped up and projected into the lumen or were free within the lumen. Mitotic activity was evident. There were also more solid areas as well as large cavities containing blood cells. The tumor was diagnosed as a malignant hemangioendothelioma. Seven other rats exposed at the same time to various doses of external radiation developed tumors at the site of exposure. There were four fibrosarcomas, one anaplastic, poorly differentiated connective tissue tumor, one squamous cell carcinoma, and one adnexal tumor of skin.