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 [localized epithelioid peritoneal mesothelioma (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 [localized epithelioid peritoneal mesothelioma (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.
Introduction
Mesothelioma is a rare neoplasm arising from mesothelial cells that line the serous membranes. The pleura is the most commonly affected site, whereas the peritoneum is the second most frequent location, accounting for approximately 10-15% of all mesotheliomas, equivalent to only a few hundred new cases annually in the United States. Unlike pleural mesothelioma, which predominantly affects men, peritoneal mesothelioma occurs in men and women with a more balanced sex distribution and typically presents in middle-aged and older adults. Asbestos exposure remains the most well-established risk factor, although its association with the peritoneal form is less pronounced than that observed in pleural mesothelioma; in addition, prior therapeutic abdominal or pelvic radiation has also been implicated.
Peritoneal mesothelioma typically presents as diffuse disease, characterized by widespread nodular or plaque-like peritoneal involvement and ascites. In contrast, the localized variant – a solitary, sharply circumscribed tumor that is histologically identical to the diffuse form but lacks serosal dissemination – is exceptionally rare and has been reported only in isolated cases. Its clinical and imaging features closely resemble those of ovarian and primary peritoneal serous carcinoma, and these entities can be reliably distinguished only through a targeted immunohistochemical panel.
We present this case report to raise awareness of localized epithelioid peritoneal mesothelioma (LEPM) as an uncommon and often under-recognized cause of a solitary pelvic mass and to demonstrate how preoperative MRI combined with a focused immunohistochemical panel can distinguish it from gynecological malignancies. This distinction is clinically important, as accurate diagnosis may significantly influence prognostic assessment and therapeutic decision-making. [Footnotes omitted]
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