Pleura & peritoneum


Peritoneal inclusion cyst

Editorial Board Member: Ricardo R. Lastra, M.D.
Editor-in-Chief: Debra L. Zynger, M.D.
David B. Chapel, M.D.
Aliya N. Husain, M.D.

Last author update: 18 July 2023
Last staff update: 18 July 2023

Copyright: 2020-2024,, Inc.

PubMed Search: Peritoneal inclusion cyst

David B. Chapel, M.D.
Aliya N. Husain, M.D.
Page views in 2023: 21,498
Page views in 2024 to date: 9,018
Cite this page: Chapel DB, Husain AN. Peritoneal inclusion cyst. website. Accessed April 15th, 2024.
Definition / general
  • Peritoneal inclusion cyst(s) are comprised of one or more cystic spaces lined by bland mesothelial cells
  • This is a broad diagnostic category of uncertain pathogenesis, ranging from small (0.1 cm) incidental unilocular cysts to florid (> 30 cm) symptomatic multilocular tumors
Essential features
  • F > M
  • Uniloculated or multiloculated cysts lined by bland mesothelial cells
  • Complete resection is mainstay of treatment
  • Recurrence rates range from 3% (in series enriched for smaller, unilocular lesions) to 50% (in series emphasizing florid, multilocular lesions) but death from disease is exceptionally rare in either scenario
  • Pathogenesis (reactive versus neoplastic) is uncertain
  • Knowledge is limited by shifting definitions and variable inclusion criteria in studies across time
  • WHO advocates for (multiloculated) peritoneal inclusion cyst(s) and discourages (benign) multicystic mesothelioma and (benign) cystic mesothelioma
ICD coding
  • ICD-10: D19.1 - benign neoplasm of mesothelial tissue of peritoneum
Clinical features
Radiology description
Radiology images

Images hosted on other servers:

Pelvic ultrasound / MRI

Pelvic ultrasound / CT

Prognostic factors
Case reports
Clinical images

Images hosted on other servers:

pelvic tumor

Gross description
Gross images

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Multiple cysts

Cystic cut section

Microscopic (histologic) description
  • 1 or multiple small cysts lined by a single layer of bland, flat to cuboidal cells
  • Delicate mesothelial papillae or buds may project into cyst lumina (Am J Surg Pathol 1988;12:737, Cancer 1989;64:1336)
  • Cysts separated by scant loose to collagenous stromal septa
  • No infiltrative invasion of underlying tissues (e.g., fat, bowel)
  • Chronic inflammation and hemorrhage are common
  • Uncommon features (Am J Surg Pathol 1988;12:737)
    • Adenomatoid tumor-like foci
    • Squamous metaplasia of the mesothelial lining
    • Hobnail change of mesothelial lining
    • Brisk acute inflammation with mesothelial denudation and granulation type change
    • Small mesothelial nests, cords and single cells in lesional stroma; often associated with brisk inflammation
  • Mesothelial nests, cords and single cells may be seen in stromal septa; often associated with brisk inflammation (termed mural mesothelial proliferation) (Cancer 1989;64:1336, Am J Surg Pathol 1986;10:844, Hum Pathol 2003;34:369)
    • Considered to be a superimposed reactive change
    • No diffuse sheet-like growth or infiltration of underlying tissues
  • Recurrences histologically similar to primary lesions (Cancer 1982;50:1615, Cancer 1989;64:1336)
Microscopic (histologic) images

Contributed by David B. Chapel, M.D.
Noninvasive growth

Noninvasive growth

Bland mesothelial lining Bland mesothelial lining

Bland mesothelial lining

Chronic inflammation Chronic inflammation

Chronic inflammation

Microscopic adenomatoid proliferation

Microscopic adenomatoid proliferation

Cytology description
Positive stains
Negative stains
Electron microscopy description
  • Well developed desmosomal attachments
  • Numerous luminal microvilli
  • Abundant cytoplasmic keratin filaments
  • Ovoid nuclei with even chromatin
  • Thin, intact basal lamina (Cancer 1982;50:1615, Cancer 1989;64:1336)
Sample pathology report
  • Pelvic mass, excision:
    • Multiloculated peritoneal inclusion cysts (6.5 cm) (see comment)
    • Comment: On microscopic examination of this multiloculated cystic lesion, individual cysts are lined by bland, flattened mesothelial cells. Mitoses are not identified. The cysts are separated by fibrous septa with patchy chronic inflammation. There is no infiltration of underlying tissues. The gross and microscopic findings are most consistent with a diagnosis of multiloculated peritoneal inclusion cyst (previously termed multicystic mesothelioma). Although peritoneal inclusion cysts are generally regarded as benign, local recurrence has been documented in up to 50% of cases. Recurrence is more common after incomplete excision. Progression to malignant mesothelioma has been reported in exceptionally rare cases. Clinical and radiographic correlation is advised with appropriate clinical follow up.
Differential diagnosis
Additional references
Board review style question #1

A 36 year old woman presents with pelvic discomfort, worsening over 6 months. Transvaginal ultrasound shows a multicystic lesion involving the cul de sac, left uterine adnexa and rectal serosa. The lesion is resected by laparoscopy. A representative H&E stained slide is shown. The cyst lining cells are positive for cytokeratin AE1 / AE3, calretinin and WT1 and negative for MOC31. The lesion is extensively sampled and no infiltrative growth is identified. Which of following is true?

  1. ~70% of such lesions harbor BAP1 mutations
  2. Complex copy number alterations are characteristic of such lesions
  3. Excision is curative; no clinical follow up is required
  4. Local recurrence affects ~50% of patients
  5. Men are affected ~4 times as often as women
Board review style answer #1
D. Local recurrence affects ~50% of patients. This is a multiloculated peritoneal inclusion cyst. The published literature and WHO blue book indicate a local recurrence rate as high as 50% (answer D). Answer A is incorrect as peritoneal inclusion cysts lack BAP1 mutations; rather, ~70% of peritoneal mesotheliomas harbor BAP1 mutations. Answer B is incorrect because peritoneal inclusion cysts lack recurrent copy number alterations. The molecular underpinnings of these lesions remain uncertain. Answer C is incorrect as the potential for local recurrence (particularly in larger multiloculated lesions) warrants clinical follow up. Answer E is incorrect as peritoneal inclusion cysts predominantly affect women, not men.

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Reference: Peritoneal inclusion cyst
Board review style question #2

Which of the following statements regarding peritoneal inclusion cysts is true?

  1. ~20% of patients progress to malignant mesothelioma over 10 years
  2. 40 - 50% of lesions harbor homozygous deletion of CDKN2A
  3. > 90% of lesions express estrogen receptor and progesterone receptor
  4. Destructive infiltration of underlying structures (e.g., omental fat) is seen in ~33% of cases and does not affect prognosis
  5. Surgical excision is considered the mainstay of treatment
Board review style answer #2
E. Surgical excision is considered the mainstay of treatment. Answer A is incorrect. Although the literature contains rare reports of apparent progression from peritoneal inclusion cyst to malignant mesothelioma, the rate of progression is much lower than 20%. Answer B is incorrect as peritoneal inclusion cysts lack CDKN2A deletions. Instead, ~40 - 50% of pleural mesotheliomas harbor CDKN2A deletion. Answer C is incorrect as ER and PR expression are rarely seen in peritoneal inclusion cysts. Answer D is incorrect as peritoneal inclusion cysts lack destructive invasion of underlying tissues. The presence of destructive invasion is consistent with a diagnosis of mesothelioma.

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Reference: Peritoneal inclusion cyst
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