What is a peritoneal inclusion cyst, particularly in women of reproductive age with a history of pelvic surgery, trauma, or endometriosis?

Peritoneal Inclusion Cyst

Definition and Pathophysiology

A peritoneal inclusion cyst (PIC) is a benign, fluid-filled, multilocular cystic lesion that forms when normal ovarian secretions become trapped between peritoneal adhesions, typically affecting women of reproductive age with prior pelvic surgery, trauma, endometriosis, or pelvic inflammatory disease. 1

• PICs are reactive mesothelial-lined cysts that develop when the peritoneum's normal fluid resorption capacity is impaired by adhesions, causing fluid accumulation in loculated spaces 2, 3
• The cyst fluid originates from a functioning ovary that continues to produce physiologic secretions, which then become entrapped rather than being normally reabsorbed 1
• PICs most commonly present in women during their third and fourth decades of life, often months to 20 years after the inciting event 2, 4

Characteristic Imaging Features

The hallmark imaging appearance on ultrasound is a cystic mass that conforms to the shape of adjacent pelvic organs and peritoneal surfaces without exerting mass effect, with the ovary typically located at the margin or suspended within the lesion. 1

• The cyst follows the contour of underlying pelvic structures rather than displacing them, distinguishing it from true ovarian neoplasms 1
• Internal septations are commonly present, representing peritoneal folds and adhesions within the fluid collection 1
• The ipsilateral ovary appears normal and functional, often visible at the periphery or within the cystic collection 1
• Transvaginal ultrasound with color Doppler demonstrates absence of internal vascularity, helping differentiate PICs from malignant lesions 2

Risk Stratification and Classification

PICs are classified as O-RADS 2 lesions (almost certainly benign, <1% malignancy risk) when they demonstrate classic imaging features. 1

• The American College of Radiology recognizes PICs as one of the classic benign extraovarian cystic lesions that can be accurately diagnosed on ultrasound 1
• PICs do not require routine follow-up when confidently diagnosed, though referral to a gynecologist may be appropriate for symptomatic management 1
• The key distinguishing features that confirm benignity include: conforming shape, lack of mass effect, normal functioning ovary, and absence of solid components or abnormal vascularity 1

Clinical Context and Associated Conditions

Prior pelvic or abdominal surgery is the most common predisposing factor, but endometriosis and pelvic inflammatory disease are also frequently associated. 2, 5

• Cesarean section, appendectomy, and other pelvic surgeries create the adhesive environment necessary for PIC formation 4, 3
• Endometriosis is often found concurrently with PICs, as both conditions involve peritoneal inflammation and adhesion formation 5
• Patients may present with pelvic pain, abdominal distension, or the lesion may be discovered incidentally during imaging for other indications 2, 4

Differential Diagnosis

PICs must be distinguished from paraovarian cysts, hydrosalpinx, cystic neoplasms, and low-grade cystic mesothelioma. 1, 4

• Paraovarian cysts are simple cysts separate from the ovary that move independently when pressure is applied with the transducer, unlike PICs which conform to surrounding structures 1
• Hydrosalpinx demonstrates characteristic endosalpingeal folds (short round projections) within a tubular fluid-distended structure 1
• Cystic mesothelioma is a rare malignant consideration that requires histopathologic confirmation, though PICs have characteristic benign imaging features 4

Management Approach

Conservative management with observation is appropriate for asymptomatic PICs with classic imaging features, while symptomatic cases may require gynecologic referral for consideration of hormonal therapy, aspiration, or surgical excision. 1, 2

• Laparoscopic excision is superior to laparotomy when surgery is indicated, with reduced blood loss, shorter hospital stay, and lower complication rates, though recurrence rates are similar between approaches 6
• Image-guided aspiration provides temporary relief but has high recurrence rates due to persistent adhesions and continued ovarian secretion 2, 6
• Hormonal contraceptives may suppress ovarian function and reduce fluid production, though evidence for this approach is limited 2

Important Clinical Pitfalls

• Do not mistake PICs for ovarian neoplasms—the conforming shape, lack of mass effect, and presence of a normal functioning ovary are key distinguishing features 1
• Do not perform unnecessary surgery for asymptomatic PICs—these are benign lesions with <1% malignancy risk when classic features are present 1
• Do not rely solely on CA-125 levels—these can be elevated in PICs and other benign conditions, particularly in the presence of endometriosis 2
• Recognize that recurrence is common after treatment—the underlying adhesive process persists even after cyst excision or aspiration 2, 6