Canine medullary carcinoma of the thyroid.

An 8-year-old male Irish Terrior dog and a male St. Bernard dog each had a thyroid medullary carcinoma. The first dog was hypocalcemic before removal and with recurrence of the neoplasm. In both dogs the neoplasms were bilateral and microscopic metastases were seen only in the regional lymph nodes. Both neoplasms were characterized by a solid alveolar pattern separated by thin vascular stroma or a linear festoon-like arrangement with thick stroma. The neoplastic cells varied little and were polygonal or round with abundant pale eosinophilic and finely granular cytoplasm. There was little mitotic activity. Both carcinomas contained amyloid. Electron microscopically, the neoplastic cells had moderate nuclear pleomorphism, well developed Golgi complexes, and characteristic membrane-bound electron-dense secretory granules. In some sections, the secretory granule membranes were fused to the cytoplasmic membrane and the granules were in the extracellular space. In one carcinoma there were linear fibrillar structures with an average width of 7.8 nanometers similar to that of amyloid fibrils.