Peritoneal inclusion cyst

Peritoneal inclusion cyst (PIC) (also known as a peritoneal pseudocyst⁵) is a type of cyst-like structure that appears in relation to the peritoneum and results from a non neoplastic reactive mesothelial proliferation.

Epidemiology

Peritoneal inclusion cysts occur almost exclusively in premenopausal women with a history of previous abdominal or pelvic surgery, trauma, pelvic inflammatory disease, or endometriosis.

Clinical presentation

Most patients with peritoneal inclusion cysts present with pelvic pain or a pelvic mass. About 10% are discovered incidentally.

Pathology

They are usually caused by accumulation of ovarian fluid that is contained by a peritoneal adhesion. The development of a peritoneal inclusion cyst depends on the presence of an active ovary and peritoneal adhesions. The normal peritoneum absorbs fluid easily. However, the absorptive capacity of the peritoneum is greatly diminished in the presence of mechanical injury, inflammation and peritoneal adhesions.

Peritoneal inclusion cysts range in size from several millimeters in diameter to bulky masses that may fill the entire pelvis and abdomen. Pathologically, the cyst results from non-neoplastic, reactive mesothelial proliferation.

Assocations

• previous abdominal trauma
• previous pelvic inflammatory disease
• previous abdominal surgery
• endometriosis

Radiographic features

They typically appear as cystic masses with septations or loculated fluid collections within the pelvis 

Ultrasound

• large, ovoid or irregular, anechoic cyst is considered characteristic ¹, but septations are not uncommon
  • size can vary from small localised collections up to large cystic masses which occupy the entire pelvis and lower abdomen
• invagination of the surrounding structures into the cyst
• lack of a discrete limiting wall
• no mural nodularity
• minimal internal debris

Other features in relation to the to ovary include:

• spider web pattern (from an entrapped ovary)

CT

May be seen as loculated fluid collection conforming to the peritoneal space with the a normal ipsilateral ovary within it or in the wall ⁵. Septations within the loculated fluid can also be encountered.

Pelvic MRI

The location of the ovary with respect to the cysts is clearly demonstrated on MRI. They tend to appear as irregular cystic masses. Signal characteristics are:

• T1: hypointense
• T2: hyperintense
• T1 C+ (Gd): no enhancement of cyst walls

Treatment and prognosis

Conservative treatment (use of GnRH analogues, oral contraceptives to suppress ovulation, pain medication) is the first line of treatment. Image-guided transvaginal fluid aspiration and sclerotherapy have been attempted with partial success ⁸.  

Surgical resection of adhesions is necessary only in selected cases. After surgical resection, the risk of recurrence is 30-50%. Peritoneal inclusion cysts have no malignant potential despite the occasional occurrence of metaplasia. 

Differential diagnosis

On imaging, a peritoneal inclusion cyst can potentially mimic a ²:

• paraovarian cyst
• hydrosalpinx
• pyosalpinx
• appendiceal mucocoele: may cause pseudomyxoma peritonei if ruptured, but is separated from ovary 

If septated, also consider:

• multicystic mesothelioma in the pelvic peritoneum
• malignant cystic ovarian neoplasm

Editorial board note

At the time of writing we are not sure how much of a overlap there is with this entity and a Multicysticmulticystic mesothelioma. If your'reyou are an expert on this we would love your help.