Peritoneal inclusion cyst (peritoneal pseudocyst, benign cystic mesothelioma) is a reactive, non-neoplastic cystic collection formed by serous fluid accumulating between pelvic/abdominal peritoneal adhesions. It is not a true cyst — it lacks an epithelial capsule and is instead described as fluid trapped between adherent peritoneal surfaces. It characteristically occurs in premenopausal women (functional ovary = fluid production) and in conditions that create adhesions such as abdominal/pelvic surgery history, endometriosis, pelvic inflammatory disease (PID), or radiotherapy. The ovary is typically embedded within the cyst in a 'spider-in-web' appearance — this is the pathognomonic finding. On US, an anechoic-hypoechoic fluid collection conforms to the shapes of peritoneal surfaces in the pelvis (molds around bowel loops, uterus, and ovary). On CT, a water-density collection is seen, and adhesion bands may appear as thin septa. On MRI, the ovary is identified within the T2 hyperintense fluid collection. Treatment for symptomatic cases is percutaneous aspiration or surgical excision, with a high recurrence rate (30-50%).
Age Range
20-55
Peak Age
35
Gender
Female predominant
Prevalence
Uncommon
Peritoneal inclusion cyst formation requires two coexisting conditions: (1) a functional ovary actively producing fluid and (2) impaired fluid resorption due to peritoneal adhesions. Under normal conditions, the ovary produces small amounts of peritoneal fluid during follicular rupture, and this fluid is reabsorbed by peritoneal mesothelial cells — net balance is maintained. Peritoneal adhesions from surgery, infection, or inflammation block free circulation of this fluid → fluid becomes trapped between adherent peritoneal surfaces → progressively accumulates and forms a cystic collection. The collection does not have a true capsule — its wall consists of adherent peritoneal surfaces lined by mesothelial cells. As the ovary continues producing fluid, the collection grows and the ovary becomes embedded within it → creating the 'spider-in-web' appearance. The fluid content is serous transudate — low protein concentration, few cellular elements. Therefore, it shows water density on CT (0-15 HU), T1 hypointense and markedly T2 hyperintense signal on MRI. Adhesions appear as thin septa within the collection — these are not mesenchymal bands but adherent peritoneal folds.
On ultrasound, the ovary appearing suspended and embedded by thin adhesions within a peritoneal fluid collection — the ovary resembles the 'spider', adhesions the 'web', and the collection the 'web spaces'. Pathognomonic finding of peritoneal inclusion cyst.
On B-mode US, the ovary is seen embedded within an anechoic-hypoechoic fluid collection in the pelvic peritoneal space — thin adhesions around the ovary create a 'spider web' and the ovary appears as the 'spider' at the center. The ovary may be normal in size and morphology or may contain functional cysts. The collection molds to organs in the pelvis — wrapping around the posterior and lateral surfaces of the uterus, cul-de-sac, and around the ovary. It is an irregularly shaped fluid collection, not a round cystic mass. Thin septa (adhesions) may be seen within the collection but no thick septa, solid component, or papillary projection is present.
Report Sentence
A __ mm anechoic fluid collection conforming to organs is seen in the pelvis with the ovary embedded within it (spider-in-web appearance); consistent with peritoneal inclusion cyst.
On color Doppler, the ovary within the collection shows normal vascular pattern — arterial and venous flow are preserved. No or minimal vascularity is seen in septa and collection wall. This finding is important in distinguishing peritoneal inclusion cyst from ovarian cystadenomas and malignant lesions — in neoplastic lesions, neovascularization (low-resistance arterial flow) is seen in septa and solid components.
Report Sentence
The ovary within the collection shows normal vascular pattern with no significant vascularity in septa or wall.
On contrast-enhanced CT, an irregularly shaped water-density (0-15 HU) collection is seen in the pelvis. The collection conforms to the contours of pelvic organs (uterus, ovary, rectosigmoid) — not appearing as a round cystic lesion. The ovary is identified as an enhancing solid structure within the collection. Thin septa may be seen within the collection but show no or minimal enhancement. Signs of peritoneal adhesions (angulation of bowel loops, mesenteric bands) may accompany. No significant wall enhancement or wall thickening is present.
Report Sentence
An irregularly shaped __ mm water-density collection conforming to organs is seen in the pelvis with the entrapped ovary; consistent with peritoneal inclusion cyst.
On T2-weighted MRI, a markedly hyperintense fluid collection is seen in the pelvis — signal intensity is CSF-like. The ovary is recognized within the collection as a structure with intermediate signal, surrounded by thin hypointense septa (adhesions). Ovarian follicles appear as small T2-hyperintense cystic structures — this finding is the MRI equivalent of the 'spider-in-web' pattern. The collection is irregularly shaped and follows the contours of pelvic organs.
Report Sentence
A markedly hyperintense irregular fluid collection is seen in the pelvis on T2 with a normal-morphology ovary and thin adhesions identified within it (spider-in-web); consistent with peritoneal inclusion cyst.
On T1-weighted MRI, the collection shows homogeneous hypointense signal — due to serous transudate content. This finding confirms the collection is not hemorrhagic or proteinaceous. Ovarian stroma shows intermediate T1 signal and can be identified within the collection. If hyperintense component is seen on T1, proteinaceous content or hemorrhage should be considered — diagnosis should be re-evaluated.
Report Sentence
The collection shows homogeneous hypointense signal on T1, not suggesting hemorrhagic or proteinaceous content.
On DWI, the collection shows no diffusion restriction — signal drops at high b-values and high ADC values (>2.0 x 10^-3 mm2/s) are seen on the ADC map. This finding confirms the collection content is free water and plays a critical role in differential diagnosis with abscess (which shows diffusion restriction from viscous purulent content).
Report Sentence
The collection shows no diffusion restriction, consistent with serous content; not suggesting abscess.
Criteria
Single or few loculated fluid collection, thin septa, no complicated findings. Usually <10 cm and asymptomatic or mild pelvic pain.
Distinct Features
Water density, homogeneous, thin septa, ovary with normal morphology. Conservative follow-up sufficient — most show spontaneous resolution.
Criteria
Large collection (>10 cm), multiple septa, significantly displaces pelvic organs. Symptomatic — pelvic pain, compression symptoms.
Distinct Features
May be confused with ovarian cystic neoplasm — but irregular shape, conforming to organs, and absence of solid component are distinguishing. Aspiration or surgery may be needed.
Criteria
Collection recurring after previous aspiration or surgery. Recurrence rate 30-50%. As long as functional ovary persists, new fluid production continues.
Distinct Features
GnRH agonists for ovulation suppression may reduce recurrence. Oophorectomy may be considered for definitive treatment but careful risk-benefit analysis needed in premenopausal patients.
Distinguishing Feature
Serous cystadenoma is a round/oval, thin-capsulated true cystic lesion — peritoneal inclusion cyst conforms to organs and is irregularly shaped. In cystadenoma, the ovary is separate from the cyst wall; in inclusion cyst, the ovary is embedded within the collection (spider-in-web).
Distinguishing Feature
Endometrioma shows T1 hyperintensity and T2 dark signal due to hemorrhagic content. Peritoneal inclusion cyst is T1 hypointense, markedly T2 hyperintense. Endometrioma is typically round and arises from ovarian parenchyma.
Distinguishing Feature
Serous carcinoma shows solid component, papillary projection, and prominent enhancement. Peritoneal inclusion cyst has NO solid component or papillary projection. Carcinoma is accompanied by ascites and peritoneal implants.
Distinguishing Feature
Abscess shows diffusion restriction on DWI (low ADC), thick rim enhancement, and prominent surrounding inflammatory changes. Peritoneal inclusion cyst shows no diffusion restriction and clinical presentation is not acute.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
6-monthPeritoneal inclusion cyst is a benign, non-neoplastic condition with no malignancy risk. Asymptomatic cases do not require treatment — US follow-up at 6-12 month intervals is sufficient. For symptomatic cases, percutaneous aspiration is the first treatment option but recurrence rate is high (30-50%). Sclerotherapy may reduce recurrence rate. Surgical excision is performed in refractory cases. Ovulation suppression with GnRH agonists may reduce recurrence. After menopause, the cyst usually shows spontaneous resolution. Biopsy is not needed.
Peritoneal inclusion cyst is a benign lesion. Percutaneous drainage or surgical excision can be performed in symptomatic patients. Recurrence is common (if adhesions persist). Differential diagnosis from ovarian malignancies is important.