A case of canine thyroid carcinoma with heterotopic ossification and extramedullary hemopoiesis

The present report describes a case of canine thyroid carcinoma with heterotopic ossification and extramedullary hemopoiesis in a 10 years old mongrel dog. Few anamnestic data were available: an apparently healthy dog was referred with the only symptom of polyphagia. A solid mass, not adherent to surrounding tissue, was palpable on the left side of the neck. The mass was surgically removed, fixed in 10% buffered formalin and send to the pathology division of our Department to be histologically examined. Grossly neoplastic mass was about 3x5 cm, well circumscribed, defined by a thin fibrous capsule. On cross section the mass had a brow-tan colour with a greyish, firm to hard, central area, grossly consistent with bone tissue. Before trimming, the mass was therefore immersed for 1 week in a decalcifying acid. Then, samples were collected, passed trough graded alcohols, clarified in xilene and paraffin embedded. From paraffin blocks, sections were obtained and stained with Haematoxylin and Eosin. Histologically, the tumor was composed of irregular, small follicular structures, nests and solid lobules of polygonal cells sustained by a variable amount of collagenous stroma. Neoplastic cells had poorly defined cells borders, moderate amount of eosinophilic cytoplasm and round to oval vescicolous nucleus with prominent nucleolus. Anisocytosis and anisokaryosis were moderate and mitotic figures ranged from 0 to 1 X HPF. Small multifocal necrotic foci and focal haemorrhages were also present scattered throughout the tumor. The central area of the tumor was composed of mature trabecular bone. Intertrabecular spaces were filled by both adipose cells and hemopoietic cells. A histological diagnosis of thyroid carcinoma with heterotopic bone formation and extramedullary hemopoiesis was posed. Heterotopic ossification is a well recognized phenomenon involving organs and tissues affected by various pathologic processes, i.e. ischemia, hematomas, degenerative changes, chronic inflammation and, less frequently, tumors. Few cases of thyroid tumors with heterotopic ossification and extramedullary hemopoiesis have been described in human species. In the canine species, the presence of focal mineralization or scattered bone formation within thyroid carcinomas has been reported but poorly documented. Scattered calcifications have been described in normal thyroid gland while ossification has been reported in a single case of thyroidal carcinosarcoma. The present report represents the first description of a canine thyroid carcinoma with wide areas of mature bone formation (heterotopic ossification) and extramedullary hemopoiesis. Mechanisms of heterotopic ossification are still unknown, however the presence of inducible osteoprogenitor cells, of heterotopic environment conductive to osteogenesis and of inductive signalling such as bone morphogenetic proteins has all been evocated in the pathogenesis of this condition.