Aggressive fibromatosis in a cat.

A case of aggressive fibromatosis (extra-abdominal desmoid) found in a 9-month-old male mixed breed cat is described. The right forearm was almost completely effaced by fibrous tissue and there were some tumours in the area from the shoulders to the mandible. These tumour-like tissues were composed of abundant collagen fibres and sparse numbers of well-differentiated fibroblasts, whereas their growing pattern was aggressive and non-encapsulated. There was dense growth of immature fibroblasts and multinucleated giant cells in some areas. Vimentin immunohistochemistry and electron microscopy suggested that the giant cells had close relation to the fibroblasts, and such areas may be the sites of cell proliferation. This case is different from nodular fasciitis and may be a proliferative disorder induced by feline oncogenic retrovirus.