Abstract
A dog developed a cervical mass, and computed tomography verified a mass surrounding the trachea with some pulmonary masses. Histopathologically, the cervical mass was composed of malignant neoplastic cells showing follicular appearance which reacted positive for thyroglobulin on immunohistochemistry. A characteristic feature of the tumor was abundant and metaplastic stromal components. Anastomosed collagenous tissues connecting to capsule of the tumor were abundant in the stroma. In parts of the collagenous tissues, mature cartilages and bones were continuously formed. There was no cellular atypia or invasion in the components. We diagnosed this case as follicular thyroid carcinoma with metaplastic stroma. This is the first case report that characterizes stromal components with chondroid and osseous metaplasia in a canine thyroid carcinoma.