Diffuse-Type Tenosynovial Giant Cell Tumor of the Elbow Mimicking a Cystic Bone Lesion: A Multimodality Imaging Case Report

Diffuse-type tenosynovial giant cell tumor (D-TGCT) is a rare, benign but locally aggressive neoplasm arising from synovial tissue.Although magnetic resonance imaging (MRI) is the preferred imaging modality, osseous involvement may obscure the synovial origin of the disease and mimic a primary bone lesion.We report a 24-year-old man with recurrent left elbow pain and intermittent swelling for approximately two years.Radiographs demonstrated a well-defined osteolytic lesion of the proximal ulna with a large joint effusion.Computed tomography (CT) showed a 1.8 × 1.6 × 1.6 cm fluid-attenuation lytic lesion with sclerotic margins, initially interpreted as a simple bone cyst.MRI subsequently demonstrated diffuse synovial thickening and enhancing proliferative synovium extending toward the proximal ulna, indicating that the apparent bone lesion represented osseous involvement associated with the proliferative synovial process.Histopathological examination after surgical biopsy and curettage showed mononuclear cells, osteoclastlike multinucleated giant cells, foamy macrophages, and abundant hemosiderin, confirming D-TGCT.Follow-up MRI showed marked postoperative regression of the intra-articular component and a small stable residual focus within the curettage cavity.This case illustrates how D-TGCT of the elbow may mimic a primary cystic bone lesion when osseous involvement dominates the initial imaging appearance.Correlation of radiographic, CT, and MRI findings is important for recognizing the synovial origin of periarticular bone erosions and directing appropriate histopathological confirmation.