Cranial Mediastinal Thyroglossal Duct Adenocarcinoma in a Dog

A 7 yr old female spayed Canaan dog was presented for evaluation of a subclinical intrathoracic mass noted during meta-static staging for a digit and lingual mass. Thoracic ultrasound and computed tomography revealed a 6.0 × 5.5 cm mass within the cranial mediastinum. The mass was septated with mixed fluid and soft-tissue structures and no evidence of vascular invasion. Narrow excision of the mass via a sternal thoracotomy was performed. Histopathology of the mass was consistent with a thyroglossal duct cyst adenocarcinoma. This is the first reported case of a thyroglossal duct cyst adenocarcinoma arising in the cranial mediastinum of a dog. There was no evidence of metastasis or recurrence 25 mo after surgery and adjunctive therapy.