Multilocular Peritoneal Inclusion Cyst (Benign Cystic Mesothelioma)

A 31-year-old woman who complained of heavy menstrual bleeding was diagnosed radiographically with multiple uterine subserosal and intramural leiomyomas. Other than being diagnosed with Graves disease 5 years earlier, she had no significant medical history. She subsequently underwent a multiple myomectomy. During the operation, multiple adhesions (suggestive of pelvic inflammatory disease) involving the right ovary were identified; these adhesions were lysed. A cystic peritoneal mass was also noted, which was removed. Gross examination of this specimen revealed a multicystic mass measuring 7.5 cm in greatest dimension. Individual cysts measured approximately 1 cm in diameter and contained serous fluid. The walls of the cysts were semitransparent and 0.1 cm thick. Microscopically, a single layer of flat cells lined the cysts (Figure 1). No associated atypia was noted. These cells stained positively for pan-keratin AE1/AE3 (Figure 2) and were negative for endothelial marker CD31. The septa consisted of loose fibrovascular tissue. No smooth muscle and no significant amount of inflammatory cells were present. A diagnosis of multilocular peritoneal inclusion cyst was rendered.Multilocular peritoneal inclusion cysts are uncommon mesothelial proliferations that tend to recur, but nevertheless are benign lesions.1,2 These lesions occur predominantly in women in the reproductive age group. The most common presenting complaints are pelvic and low abdominal pain, but sometimes the lesions are incidental findings at laparotomy. Most of the patients have a history of a previous pelvic operation, endometriosis, and/or pelvic inflammatory disease.1,2 Microscopically, a single layer of flat to cuboidal benign mesothelial cells typically lines multilocular peritoneal inclusion cysts. Occasionally, squamous metaplasia, hobnailing, small papillae, and cribriform patterns may be appreciated.1 One case with a microcystic pattern and hyaline globules mimicking a yolk sac tumor was described.3 The septa consist of loose fibrovascular connective tissue with a sparse inflammatory infiltrate. The inflammatory component in some cases may be marked and accompanied by fibrin, granulation tissue, and recent and old hemorrhage. These clinicopathologic features have led some authors to suggest that these lesions are reactive.2 Still others believe that they are neoplastic in origin.4Although there is a long differential list for multilocular peritoneal inclusion cysts, they are most likely to be confused with multilocular cystic lymphangiomas. Unlike patients with multilocular peritoneal inclusion cysts, patients with lymphangiomas are usually male (75%) and are predominantly younger than 5 years (60%).1,2 Microscopically, clues pointing toward lymphangiomas include the presence of lymphoid aggregates and smooth muscle in their walls. Since endothelial cells line lymphangiomas, problematic cases can be solved using immunohistochemistry.