Histiocytic sarcoma with a granuloma-like component occurring in a large colony of Sprague-Dawley rats.

Tumors from a large colony of over 2000 Sprague-Dawley rats were reviewed by a panel of pathologists under the auspices of Universities Associated for Research and Education in Pathology, In the course of this study, a distinct malignancy in 105 rats was segregated and given the name "histiocytic sarcoma." This report describes the histologic features and anatomic distribution of the tumor, along with its incidence by age and sex of the rat. The neoplasm is characterized by a spectrum of histologic patterns, one extreme resembling a sarcomatous process and the other a granulomatous process. The exact morphologic composition varied from animal to animal, as well as in tumors from the same animal. The sarcomatous pattern is composed of sheets of uniform cells, occasionally exhibiting phagocytosis. The granulomatous pattern contains epithelioid histiocytes, central areas of tumor necrosis surrounded by palisading cells, and multinucleate giant cells. The liver is the organ most commonly containing histiocytic sarcoma, both in animals with multiple tumors and in those with involvement of only one organ. Hepatic invasion is typified by expansion of the portal areas and infiltration of the sinusoids by tumor cells. The lung is the second most common site. The distribution of pulmonary tumors is bronchiolocentric and angiocentric. Other common sites included the lymph node and spleen, along with large soft tissue tumors of the mediastinum and retroperitoneum. Although occasional previous reports apparently have described a similar or identical neoplasm under varying names, no large group of tumors of this unique type in rats has been studied and characterized previously.