Non-functional thyroid cystadenoma in three boxer dogs.

Background: Thyroid neoplasia is a common endocrine neoplasm in dogs. The boxer is one of the reported breeds predisposed to malignant thyroid neoplasia. However, the association between thyroid neoplasia, malignancy and breed should be considered with caution.
Cases Presentation: This article describes the presentation, clinical pathological findings, computed tomographic (CT) imaging findings and histopathological features of benign cystic thyroid tumour (cystadenoma) diagnosed in three boxers. These three dogs were presented for investigation of unilateral (n = 2) or bilateral (n = 1) cervical masses with no associated clinical signs of thyroid dysfunction. In each case, post-contrast CT scan identified a large, lateralised, non-invasive, well-defined homogeneous cystic structure with a hyperattenuating contrast-enhancing capsule of suspected thyroid origin displacing the surrounding cervical tissues. Ultrasound-guided fine needle aspiration of the cysts yielded fluid with a high thyroxine concentration in each case. Histopathology was consistent with thyroid cystadenoma in all cases. One dog was concurrently diagnosed with oral melanoma and euthanased. Two dogs underwent surgical excision with one lost to follow-up after 36 months and the other euthanased after 16 months following diagnosis of mast cell tumour.
Conclusions: To the authors' knowledge, this is the first detailed report of non-functional benign thyroid cystadenoma in dogs and provides relevant information about case management for this type of tumour. The presence of a large cystic structure associated with benign non-functional thyroid neoplasia may be a condition to which boxer dogs are predisposed.