Localized Epithelioid Peritoneal Mesothelioma: A Rare Mimicker of Gynecological Malignancy

Localized epithelioid peritoneal mesothelioma (LEPM) is a rare variant of peritoneal mesothelioma that presents as a solitary, circumscribed mass rather than with the diffuse serosal spread that characterizes the far more common diffuse form. Because it may arise in the pelvis and be accompanied by ascites and a mildly elevated carbohydrate antigen (CA)-125 level, it is readily mistaken for a gynecological or primary peritoneal malignancy. Although it is generally regarded as less aggressive than the diffuse form, its clinical behavior can be unpredictable. We report the case of a 76-year-old woman in whom a 5 cm pelvic mass was detected incidentally on transvaginal ultrasound and was initially suspected to be a gynecological malignancy. Preoperative MRI demonstrated clear cleavage planes between the mass and the uterus and left ovary, indicating a non-gynecological origin, and surgery confirmed the presence of a tumor on the antimesenteric border of the small bowel. Histopathology, supported by a targeted immunohistochemical panel (showing calretinin, WT1, D2-40, and CK7 positivity and loss of BAP1), established the diagnosis of LEPM. Despite an apparently localized presentation and complete resection, early peritoneal recurrence developed on follow-up imaging, prompting adjuvant systemic therapy and cytoreductive surgery (CRS). This report underscores that LEPM should be considered in the differential diagnosis of a solitary pelvic mass, highlights the importance of cross-sectional imaging combined with a specific immunohistochemical panel for distinguishing it from gynecological tumors and gastrointestinal stromal tumor (GIST), and emphasizes the need for close follow-up even when the disease appears localized.