A large peritoneal inclusion cyst in a postmenopausal female without risk factors.

Peritoneal inclusion cysts (PICs) are rare, benign cystic lesions most often seen in females of reproductive age with a history of peritoneal injury. This case presents a 55-year-old postmenopausal female with no known risk factors who developed a large PIC, causing abdominal pain, fullness, and distention. Imaging, including CT and MRI, revealed a lobulated, fluid-filled mass in the left peritoneal cavity, initially suspected to be a lymphangioma. MRI confirmed the presence of a multilocular cyst with enhancing septa. Surgical exploration identified the cyst as a PIC involving the jejunal mesentery, with final pathology confirming the benign nature of the lesion. While PICs are typically associated with premenopausal females and peritoneal trauma, this case highlights their potential occurrence in postmenopausal age groups without prior insults. The ability to recognize characteristic imaging features-such as cystic lesions with enhancing septa on CT and MRI-can help differentiate PICs from other cystic abdominal masses, including lymphangiomas and neoplastic cystic masses.