Metastatic thyroid carcinoma in the appendicular skeleton and tibial plateau leveling osteotomy site of a dog

A 13-year-old spayed female Labrador retriever cross dog was referred to the Foster Hospital for Small Animals at Tufts University for evaluation of a metastatic carcinoma discovered at the site of a previous tibial plateau leveling osteotomy (TPLO). The dog was previously evaluated at the primary care veterinarian for a complaint of lameness, and radiographs of the previous TPLO site revealed bony lysis associated with the TPLO plate. Surgical exploration of the site by the primary veterinarian provided evidence of osteolysis. The lytic area of the proximal tibia was biopsied, and a metastatic carcinoma was confirmed. The patient was referred for further evaluation. Following consultation and examination, a full body computed tomography (CT) scan was performed to determine the source of the carcinoma. A well-defined soft tissue attenuating mass effacing the right thyroid gland was located, along with 3, well-defined soft tissue attenuating nodules within the pulmonary parenchyma, consistent with metastatic disease. The previously diagnosed osteolytic, aggressive bone lesion of the proximal left tibia was visualized. Following the CT scan, palliative left hind-limb amputation via coxofemoral disarticulation was performed. Histological examination of the hind limb revealed neoplastic epithelial cells admixed with reactive bone. Neoplastic cells were arranged in packets with rare colloid-filled microfollicles consistent with a diagnosis of metastatic thyroid carcinoma. To the authors' knowledge, this is the first clinical report of metastatic thyroid carcinoma in the appendicular skeleton and TPLO site of a dog. Key clinical message: Our findings emphasized that thyroid carcinoma may metastasize to the appendicular skeleton, and causes other than osteomyelitis or implant-associated osteosarcoma should be considered when evaluating osteolytic lesions at a TPLO site.