Proximal-Type Epithelioid Sarcoma with Unusual Morphology: A Diagnostic Challenge with Detailed Immunohistochemical Workup

Background:An uncommon and aggressive soft tissue sarcoma, Epithelioid sarcoma (ES) has a significant risk of recurrence and metastasis. The proximal variant overlaps with other epithelioid malignancies, it can be diagnostically challenging particularly in young patients. Case Presentation:We report a single-patient case of a 14-year-old male with a painful deep-seated calf mass. Trucut biopsy showed deceptively bland epithelioid–rhabdoid cells with lymphocytic admixture. After Immunohistochemistry (IHC), lesion was reported as a polygonal cell neoplasm on biopsy. Wide local excision was performed, morphology showed myxoid matrix making it more difficult to diagnose and extended IHC demonstrated PanCK and EMA positivity with complete loss of INI1, while S100, GFAP, CD34, Desmin, SALL4, and CD138 were negative, confirming proximal-type epithelioid sarcoma. Conclusion:This case emphasizes that proximal-type epithelioid sarcoma may present with deceptively bland histology and unusual myxoid stromal changes. Definitive diagnosis requires extended immunohistochemical panel, particularly assessment of INI1 loss.