Spontaneous fibrous histiocytic neoplasms in rats.

A total of 85 spontaneous rat fibrohistiocytic tumours were evaluated histologically and assessed for the presence or absence of metastases. The overall incidence in controls from 2-year carcinogenicity studies was 2.7%. The tumours occurred principally in the subcutaneous and deep soft tissues, and generally appeared after 18 months of age. Four histological types were recognized: histiocytic (17%), pleomorphic (33%), cellular (17%) and very fibrous (33%). Histiocytic tumours were highly malignant, and most produced metastases. Pleomorphic and cellular neoplasms occasionally produced metastases and must be regarded as potentially malignant. Very fibrous lesions were essentially benign. The close resemblance, both histologically and biologically, between rat and human fibrohistiocytic neoplasms supports the use of the fibrohistiocytic concept in laboratory-animal pathology. Study of these rat tumours may provide insight into the development of human fibrohistiocytic neoplasms.