Tenosynovial giant cell tumors lacking giant cells: report of diagnostic pitfalls.

Tenosynovial giant cell tumors are a group of neoplastic disorders that involve synovium-lined tendon sheaths, synovial joints, and adjacent soft tissue. They are divided into localized and diffuse subtypes. TSGCTs have well-established clinical and histological diagnostic criteria; however, the subtypes and nomenclature can be confusing. They also pose diagnostic challenges when they occur in atypical locations or without multinucleated giant cells. With the possibility for systemic targeted therapy in relapsing pigmented villonodular tenosynovitis and diffuse-type giant cell tumor, accurate diagnosis and subtyping of TSGCTs is increasingly important. We report two cases of TSGCTs in order to elucidate the diagnostic nomenclature, clinicopathological features, differential diagnosis, and diagnostic pitfalls. Recent advancements in the pathogenesis and targeted therapy of TSGCTs are also discussed.