Indolent Abdominal Wall Round Cell Sarcoma With EWSR1-PATZ1 Fusion: A Case Report and Literature Review.

Soft tissue sarcomas with Ewing Sarcoma Breakpoint Region 1-POZ/BTB and AT Hook Containing Zinc Finger 1 (EWSR1-PATZ1) gene fusion represent a recently recognized subgroup of "round cell sarcomas with EWSR1-non-ETS fusions." These tumors exhibit diverse morphologic features and a polyphenotypic immunoprofile, often co-expressing neural and skeletal muscle markers. Their clinical behavior ranges widely, from indolent to highly aggressive. We report the case of a 57-year-old man presenting with a 6.5 cm right abdominal wall mass incidentally discovered on CT imaging performed for choledocholithiasis. Ultrasound-guided biopsy revealed a low-grade spindle cell neoplasm, and the patient subsequently underwent marginal excision. Microscopic examination showed solid and pseudocystic architecture with thick fibrous septa, composed of round and spindle cells with eosinophilic or clear vacuolated cytoplasm and vesicular chromatin. Immunohistochemistry displayed positivity for CD99, BCL-2, and patchy desmin positivity. Targeted gene fusion analysis confirmed EWSR1-PATZ1 fusion. Given the long-standing, asymptomatic nature of the mass, the patient was managed with clinical surveillance and remains disease-free for three years post-excision. Due to the rarity of these tumors, their biologic behavior and optimal management remain uncertain, highlighting the need for individualized treatment strategies and long-term follow-up.