Metastasizing fibrosarcoma in a Wistar rat--case report

A 19-month-old male control Wistar rat from a 30-month inhalation study showed a subcutaneous greyish-white mass extending from the throat to the thoracic cavity. The rat had been euthanized because of its poor general condition. Histologically, the mass was diagnosed as a fibrosarcoma infiltrating the masseter muscle with metastases in the lungs, liver and heart. The primary tumour was characterized by fusiform spindle cells producing various amounts of interlacing bundles of collagen. The cells formed a characteristic herringbone pattern and mitotic figures were frequent. The histological parameters of the metastases were practically identical to those seen in the primary tumour. The diagnosis was confirmed by trichrome staining and positive immunostaining for vimentin and was differentiated from leiomyosarcomas by its negative immunostaining for desmin, from schwannomas by its negative immunostaining for S-100 and from malignant fibrous histiocytomas by the absence of giant cells. The incidence of fibrosarcomas in Wistar rats is very low (up to 3%) and metastasis is rarely observed.