滑膜肉瘤是一种罕见的软组织恶性肿瘤，主要发生在腱鞘和关节附近。腹部原发部位极为罕见，临床表现非特异。我们报告了一例无医史的年轻女性患者，出现急性腹盆疼痛。超声心动图显示右侧囊性实质性卵巢性质的肿块，直径为7cm。进行腹腔镜检查和腹膜活检。组织学检查配合免疫组化学方法，确诊为胃肠道间质瘤。患者被转诊到外科科室，在进行常规实验室分析和计算机断层扫描后，建议手术治疗。手术过程中发现肠系膜肿块侵犯大网膜和阑尾壁。病理学检查配合免疫组化学结果证实，肠系膜单相滑膜肉瘤侵犯阑尾壁，手术切缘呈阳性。患者接受了化疗并取得了良好的疗效。目前，患者在术后9个月没有复发。通过这个病例报告，我们旨在讨论肠系膜滑膜肉瘤的发现方式，其模仿卵巢恶性肿瘤，强调临床病理特征和鉴别诊断。© 2023. 日本外科学会。

Synovial sarcoma is an uncommon soft tissue malignancy that mainly occurs near tendon sheath and bone joints. Primary intra-abdominal location is exceedingly rare and characterized by non-specific clinical signs.We report the case of a young female without medical history who presented with acute abdominopelvic pain. Ultrasound echography revealed a right mass measuring 7 cm in greater diameter cystic with solid areas, likely of ovarian origin. A coelioscopy with peritoneal biopsies was performed. Histological examination with immunohistochemistry concluded the diagnosis of GIST. The patient was referred to the surgery department and after laboratory routine analysis and computed tomography, the patient was proposed to surgical management. Per-operative findings revealed a mesenteric mass locally invading the greater omentum and the appendicular wall. Pathological examination with immunochemistry confirmed the diagnosis of mesenteric monophasic synovial sarcoma invading the appendicular wall with positive surgical margins. Chemotherapy was proposed with a good response. Our patient is free from disease 9 months later.We aimed through this case report to discuss mesenteric presentation monophasic SS, mimicking ovarian malignancy, emphasizing clinicopathological features and differential diagnoses.© 2023. Japan Surgical Society.