Aortic body tumour with adjacent ectopic thyroid tissue in a dog.

A 12-year-old neutered male Husky dog had a neoplasm at the base of the heart which did not invade surrounding tissues. Microscopically, the neoplasm was composed of nests and sheets of polyhedral cells subdivided into lobules by trabeculae of fine fibrovascular stroma. Adjacent to the neoplasm was a rim of ectopic thyroid tissue that appeared histologically normal. The possible differential diagnoses for the neoplasm were aortic body tumour, ectopic thyroid tumour and ectopic parathyroid tumour; the ultrastructural characteristics revealed it to be an aortic body tumour.