Peritoneal Inclusion Cyst Mimicking Peritoneal Carcinomatosis: A Benign Lesion Posing a Malignant Dilemma

Peritoneal carcinomatosis is typically associated with advanced malignancy and is most often identified on imaging by multifocal peritoneal and omental involvement. However, benign conditions may closely mimic these findings, leading to diagnostic uncertainty. Peritoneal inclusion cysts are benign, multiloculated cystic lesions arising from reactive mesothelial proliferation, often in the context of prior inflammation or adhesions, and may resemble peritoneal carcinomatosis on cross-sectional imaging. We describe a case of a 68-year-old postmenopausal woman with a history of diverticulitis who presented with chronic non-offensive vaginal discharge and intermittent hematuria. Pelvic examination revealed no abnormal findings, and serum CA-125 was within the normal range. Pelvic magnetic resonance imaging showed a loculated fluid collection in the pouch of Douglas, and contrast-enhanced computed tomography identified multiple peritoneal and omental nodules, raising concern for peritoneal carcinomatosis. Diagnostic laparoscopy and subsequent total abdominal hysterectomy with bilateral salpingo-oophorectomy and infracolic omentectomy were performed. Histopathological evaluation confirmed benign multicystic peritoneal inclusion cysts without atypia. This case underscores the diagnostic challenge when benign peritoneal pathology mimics malignancy. When tumor markers are normal and no primary malignancy is identified, careful integration of clinical assessment, imaging features, and histopathologic confirmation is essential to avoid unnecessary radical intervention.