Ectopic Thyroid Carcinoma with Metastases in a Beagle Dog

Abstract
A 10.25-year-old female beagle had an invasive neoplasm at the base of the heart which metastasized to the lungs, pancreas, and kidney. The neoplasm in these locations was composed of clusters of polyhedral cells surrounded by fine fibrovascular stroma. The differential diagnoses were malignant chemodectoma, ectopic parathyroid carcinoma, and ectopic thyroid carcinoma. Based on ultrastructural features, this tumor was an ectopic thyroid carcinoma.