Multiorgan metastases with massive bone involvement of a medullary thyroid carcinoma in a dog

A 10-year-old mixed-breed male dog was referred for a subcutaneous mass on the ventral neck. Based on total-body computed tomography (TBCT), the mass was located in the left thyroid lobe. Further alterations included enlargement of the ipsilateral mandibular and prescapular lymph nodes (LNs). Surgical excision of the mass and enlarged LNs was performed. Histopathology and immunohistochemistry were consistent with a medullary (C-cell) thyroid carcinoma, with no evidence of nodal metastases. Surgery was considered curative, and no medical treatment was provided. Periodic follow-up rechecks were unremarkable. After 18 months, the dog exhibited lethargy, vomiting, anorexia, and hind leg stiffness. TBCT revealed polyostotic osteopathy, and cytology suggested a metastatic endocrine carcinoma. Due to the dog's poor clinical condition and prognosis, the owner elected euthanasia, and a necropsy was performed. Based on gross pathology, histopathology, and immunohistochemistry, multiple metastases of the previous thyroid carcinoma were diagnosed, involving the occipital bone, multiple vertebrae, left sacral wing, fourth right rib, left scapula, left humerus, intrathoracic LNs, lung, spleen, and adrenal glands. This report describes a case of medullary thyroid carcinoma with distant multiorgan metastases and massive bone involvement after a disease-free interval of 18 months.