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: 8 January 2021
Last staff update: 7 April 2023 (update in progress)

Copyright: 2020-2023,, Inc.

PubMed Search: Multicystic mesothelioma [TIAB]

David B. Chapel, M.D.
Aliya N. Husain, M.D.
Page views in 2022: 7,498
Page views in 2023 to date: 5,904
Cite this page: Chapel DB, Husain AN. Peritoneal inclusion cyst. website. Accessed May 29th, 2023.
Definition / general
  • Multicystic mesothelioma is a bland mesothelial proliferation of uncertain pathogenesis
  • Local recurrences are common but death from disease is exceptionally rare
Essential features
  • F > M
  • Multiloculated cysts lined by bland mesothelial cells
  • Surgical resection is mainstay of treatment
  • Approximately 50% recur but death from disease is exceptionally rare
  • Benign multicystic mesothelioma
  • Benign cystic mesothelioma
  • Cystic mesothelioma
  • Multiloculated peritoneal inclusion cyst
  • Peritoneal inclusion cyst(s)
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

Images hosted on other servers:

Multiple cysts

Cystic cut section

Microscopic (histologic) description
  • Numerous 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 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 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:
    • Multicystic mesothelioma (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 multicystic mesothelioma (also termed multiloculated peritoneal inclusion cyst). Although multicystic mesothelioma is generally regarded as a benign lesion, 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 followup.
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. Approximately 70% of such lesions harbor BAP1 mutations
  2. Complex copy number alterations are characteristic of such lesions
  3. Excision is curative; no clinical followup is required
  4. Local recurrence affects approximately 50% of patients
  5. Men are affected approximately four times as often as women
Board review style answer #1
D. Local recurrence affects approximately 50% of patients. This is multicystic mesothelioma.

Comment Here

Reference: Multicystic mesothelioma
Board review style question #2

Which of the following statements regarding multicystic mesothelioma is true?

  1. 40 - 50% of lesions harbor homozygous deletion of CDKN2A
  2. Approximately 20% of patients progress to malignant mesothelioma over 10 years
  3. Destructive infiltration of underlying structures (e.g., omental fat) is seen in approximately 33% of cases and does not affect prognosis
  4. Over 90% of lesions express estrogen receptor and progesterone receptor
  5. Surgical excision is considered the mainstay of treatment
Board review style answer #2
E. Surgical excision is considered the mainstay of treatment

Comment Here

Reference: Multicystic mesothelioma
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