Extra-uterine Endometrial Stromal Sarcoma of the Stomach Mimicking a Gastrointestinal Stromal Tumour: A Case Report

Extra-uterine endometrial stromal sarcoma (EESS) is a rare mesenchymal tumour, and primary gastric involvement represents an exceptionally unusual presentation. Because such tumours present as ulcerated submucosal masses, they are easily misdiagnosed as gastrointestinal stromal tumours (GIST), making accurate, timely recognition critical for appropriate management. A 53-year-old woman presented with a five-day history of epigastric pain, low-grade fever, and malaise. Computed tomography (CT) imaging revealed an 11 cm vascular mass on the greater curvature of the stomach with splenic-hilum and transverse-colon contact, without evidence of local invasion. Upper gastrointestinal endoscopy revealed a friable ulcerated lesion, and biopsies suggested a spindle-cell neoplasm favouring GIST. At laparotomy, the tumour was found to infiltrate the stomach, splenic hilum, colon serosa, and omentum. An en bloc partial gastrectomy, splenectomy, segmental colectomy, cholecystectomy, and omentectomy were performed; final pathology showed microscopic involvement of the gastric radial soft-tissue margin. Histology demonstrated a primary gastric low-grade ESS with a vascular pattern and low mitotic index, while immunohistochemistry showed diffuse CD10/ER/PR positivity and absence of c-KIT, DOG-1, desmin, S100, SOX10, and STAT6, excluding GIST, leiomyosarcoma, PEComa, solitary fibrous tumour, and schwannoma. Targeted RNA-based sequencing identified a canonical JAZF1::SUZ12 fusion, confirming low-grade EESS. Given the tumour’s low-grade, hormone-receptor-positive biology and microscopic radial margin involvement, adjuvant endocrine therapy with letrozole (2.5 mg daily) was initiated. At the six-month follow-up, the patient remained asymptomatic with no radiological evidence of recurrence. Primary gastric ESSS remains an exceptional finding and is easily mistaken for GIST. Accurate diagnosis and optimal management depend on comprehensive histology and immunohistochemistry, supported by fusion-gene testing, to guide individualised surgical and adjuvant management. Given the tumour’s potential for very late relapse, prolonged surveillance is warranted despite the favourable short-term outcome.