Benign multicystic peritoneal mesothelioma presenting as a retroperitoneal pelvic mass in a young woman: a case report.

Benign multicystic peritoneal mesothelioma is a rare mesothelial neoplasm with fewer than 200 reported cases, typically arising on peritoneal or pelvic surfaces in premenopausal women. We describe a 21-year-old woman with a 2-week history of left lower abdominal pain. Magnetic resonance imaging showed a multiloculated cystic retroperitoneal mass adherent to the sigmoid colon, with normal ovaries and uterus. CA-125 was mildly elevated. At laparotomy, the mass was confirmed to be retroperitoneal without ovarian involvement, requiring en-bloc excision with sigmoid colectomy and primary anastomosis. Histopathology showed flat-to-cuboidal mesothelial cells with a hobnailing pattern in fibromyxoid stroma; immunohistochemistry was positive for calretinin and WT-1, and negative for SALL4, GATA3, and CD31, confirming benign multicystic peritoneal mesothelioma. At 6-month follow-up she remained asymptomatic with no recurrence on imaging. This case highlights BMPM as a mimicker of primary retroperitoneal neoplasms and the potential need for bowel resection to achieve complete excision.