Anaplastic thyroid carcinoma in a brush‐tailed rock‐wallaby (Petrogale penicillata)

Background Neoplasia is considered to be rare in macropods. Anaplastic thyroid carcinoma (ATC) also known as undifferentiated or giant cell carcinoma, is a rare but aggressive and lethal solid tumour reported to affect humans, dogs, cats, racoons and birds. It is derived from poorly differentiated follicular cells and lacks the characteristic architectural pattern of arrangement of tumour cells. ATC has not previously been reported in macropods. Case Report A brush‐tailed rock‐wallaby (Petrogale penicillata) was presented for a mass on the ventral neck. A clinical diagnosis of thyroid carcinoma was suspected based on radiology, ultrasound and cytology. Other than palliative care, treatment was declined. Four months later the wallaby was found dead and submitted for necropsy. Gross examination and histopathology demonstrated a unilateral ATC with vascular neoplastic emboli and distant metastases to the heart, lungs and liver. Conclusion The clinical signs associated with thyroid tumours may be the result of localised growth and expansion of the thyroid, metastatic disease or a combination of these effects. Most thyroid tumours are nonfunctional. Based on the lack of typical clinical signs associated with functional thyroid tumours, in this case, we conclude that the thyroid tumour in this wallaby was likely to be nonfunctional. The cause of death in this wallaby was likely due to the heavy tumour burden with compromised cardiorespiratory function exacerbated by dorsoventral compression of the larynx. More study is needed to better understand thyroid neoplasia in macropods.