Peritoneal cystic mesothelioma is a rare benign tumor that arises from mesothelial cells of the peritoneum and is seen more commonly in young and middle-aged women. It tends to have a local recurrence. Its common location is in the pelvis and may extend into the abdomen, although it can affect any part of the peritoneum or omentum.

The etiology of this tumor is not known. Unlike its malignant counterpart, the cystic variety of mesothelioma has no relation to asbestos exposure. Chronic peritoneal irritation and previous laparotomy are implicated risk factors. Confirmatory diagnosis is usually achieved by immuno-histochemical and electron microscopic methods.

Imaging findings:
Ultrasound: This is usually the first imaging modality that is used when a woman presents with either abdominal fullness or a palpable tumor. The presence of a thin-walled cyst attached to the peritoneum on ultrasound should arouse suspicion for this rare entity. The ultrasound findings are quite non-specific and include-
1. A complex cystic mass that may show either single or multiple thin-walled cysts with septations.
2. The septations are thin-walled (less than 5 mm in thickness) and show no nodularity.

CT and MRI also show a similar non-specific appearance of the cystic mass but allow for a more complete assessment of the extent of the disease.

Differential diagnosis: It is important to distinguish this tumor from the following other cystic masses-
1. Ovarian mucinous cystadenoma/carcinoma: A predominantly pelvic mixed solid-cystic mass with involvement of the ovary. Additional findings of ascites, omental thickening and serosal deposits may also be seen.
2. Loculated pelvic fluid or a seroma: seen as a large cystic collection with the patient giving a history of peritoneal surgery. 
3. Peritoneal inclusion cyst: Also known as peritoneal pseudocyst or inflammatory cysts of the pelvic peritoneum.  The development of this lesion requires the presence of pelvic adhesions [usually following surgery] and functioning ovaries. The normal ovary produces an exudates that forms a substantial component of the peritoneal fluid.  If the peritoneum is damaged or inflamed it fails to resorb the fluid which may accumulate in locules formed by pelvic adhesions, resulting in a complex multicystic mass. The normal ovaries may be trapped within the mass. This condition forms the closest differential for benign cystic mesothelioma and can be differentiated from the latter on histology by the absence of smooth muscle cells within the septa.
4. Pseudomyxoma peritonei: This condition occurs secondary to a mucin producing neoplasm and has a predominant solid component and may involve the viscera.
5. Abdominal lymphangioma: This occurs in children and young adults with a pelvic location being uncommon. Demonstration of fat may be helpful.
6. Malignant mesothelioma: Occurs after exposure to asbestos and has a solid component.