Navigating through the diagnostic minefield of clear cell sarcoma of soft tissue: Lessons from a challenging case with literature review.

Clear cell sarcoma of soft tissue (CCSST) is a rare soft tissue sarcoma occurring in young adults with a predilection for deep soft tissues of the distal extremities. Its overlapping morphology and immunohistochemical profile pose a diagnostic challenge. Herein, we present a rare case of CCSST with a unique immunohistochemical profile arising in an uncommon location. A 36-year-old male presented with a progressively increasing painful swelling in the left supraclavicular region for the last 2 months. Positron emission tomography showed FDG-avid lesions in the left supraclavicular and scapular regions. Fine needle aspiration cytology (FNAC) followed by core needle biopsy was performed. The cytology smears showed predominantly discohesive sheets of polygonal tumor cells with prominent macronucleoli in a vacuolated background. On immunocytochemistry, tumor cells showed positivity for vimentin, HMB45, and S100, confirming the diagnosis of CCSST. Histopathological examination showed sheets of similar tumor cells that were positive for vimentin, HMB45, melan A, CD38, and CD138, representing a potential diagnostic pitfall in the index case. The index report, besides highlighting the characteristic pathologic features of CCSST and its mimics, is unique due to the diffuse positivity of the tumor cells for CD38 and CD138. It is imperative to be aware of this diagnostic pitfall as it may muddle the diagnosis of CCSST.