Epithelioid Enigma: Proximal‐Type Epithelioid Sarcoma in Cyst Fluid Cytology

Abstract

Epithelioid sarcoma (ES) is a rare, aggressive soft tissue malignancy with substantial morphologic overlap with other epithelioid neoplasms, creating diagnostic difficulty in cytology specimens. We report proximal-type ES identified in cyst fluid from a lesion clinically interpreted as an epidermal inclusion cyst or abscess. A 43-year-old man presented with right thigh pain and a small cystic lesion on ultrasound. The fluid specimen was hypocellular and contained malignant epithelioid to rhabdoid cells with marked atypia, prominent nucleoli, and focal perinuclear clearing. Tumor cells expressed keratins and showed complete loss of INI1/SMARCB1, while claudin-4, MOC31, desmin, and SOX10 were negative. A concurrent core biopsy confirmed proximal-type ES. This case expands the recognized cytologic spectrum of proximal-type ES and highlights that a hypocellular cyst fluid specimen may nevertheless provide diagnostically useful material when morphology is integrated with a focused immunohistochemical panel.