Maxillary fibrosarcoma in an alpaca
History: Histopathology dated 9/30/08 from another laboratory revealed `fibroma with osseous metaplasia`. Mass appears invasive into the bone, periodontal issues, and buccal mucosa, raising concerns aboutneoplasia.
Gingiva: Within the superficial and deep submucosa and invading the alveolar bone is a poorly-delineated, unencapsulated, invasive neoplasm composed of stellate cells embedded in abundant fibrovascular stroma. Neoplastic cells have distinct cell borders, small amounts of eosinophilic cytoplasm, an elongate nucleus with finely stippled chromatin and indistinct nucleoli. There is mild anisocytosis and anisokaryosis. No mitotic figures were present in ten 400X fields. The neoplasm contains spicules of immature woven bone whihc is rarely mineralized. The superficial submucosa is multifocally infiltrated by aggregates of small numbers of neutrophils.
The rather benign appearance of the fibroblasts in this neoplasm may lead to a diagnosis of fibroma with osseous metaplasia, ossifying fibroma, or fibrous dysplasia. Unfortunately in this case time ultimaetly revealed that this neoplasm was locally invasive, thereby warranting the diagnosis of fibrosarcoma with osseous metaplasia. this neoplasm has a histologic appearance and biologic behavior similar to the canine maxillary fibrosarcoma which arises from the periosteum and causes lysis of the maxilla. It is a low grade fibrosarcoma with little chance of metastasis. They are less cellular than dermal fibrosarcomas and lack interwoven bundles of collagen.
The distinction between fibrosarcoma with osseous metaplasia and osteosarcoma lies in the presence of normal osteoblasts which line the trabeculae of woven bone. In osteosarcomas the osteoid is lined by neoplastic cells and tends to be less nodular in appearance.
Thompson KG, Pool RR. Tumors of bones. Chapter 5 In Meuten DJ ed. Tumors of Domestic Animals 4th ed. Blackwell 2002: p245-318.