Feline anaplastic giant cell adenocarcinoma of the thyroid

An 8-year-old castrated, male, domestic short-haired cat had anaplastic giant cell adenocarcinoma in the thyroid gland. The cat had difficulty in breathing and swallowing because of a rapidly growing mass in the left thyroid region that partially enclosed the trachea and esophagus and had evidence of diffuse discrete interstitial pulmonary metastases. The neoplasm, which was locally invasive, was formed by groups of pleomorphic cells arranged in an endocrine-like pattern. The cells were found, spindle, fusiform or irregular; some were large and multinucleated with up to 50 nuclei. Metastases were in the lungs, pleura and regional lymph nodes. Microscopically, both thyroid glands were involved; areas of transition from a well differentiated papillary adenocarcinoma to an anaplastic form were seen.