A unique coincidence: Concurrent oncocytic thyroid Carcinoma and tenosynovial giant cell tumor: A case report

Abstract

Oncocytic thyroid carcinoma (OTC) and tenosynovial giant cell tumor (TGCT) are rare neoplasms with distinct biological behavior, and their coexistence has not been previously well documented. We report a rare case of concurrent recurrent OTC and diffuse-type TGCT (TGCT-D) in a 45-year-old man who presented with a rapidly enlarging soft-tissue mass involving the left ankle and foot. 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) demonstrated intensely FDG-avid lesions in the thyroid bed consistent with recurrent disease, as well as a hypermetabolic infiltrative musculoskeletal lesion in the left ankle and foot, initially suspected to represent metastatic spread. However, histopathological examination of the ankle lesion established the diagnosis of TGCT-D. This case illustrates an important diagnostic pitfall in oncology imaging: FDG-avid musculoskeletal lesions in patients with known malignancy do not necessarily represent metastases. Careful integration of clinical information, cross-sectional imaging, particularly MRI, and histopathological confirmation is essential to avoid misdiagnosis and to ensure appropriate management.