Testis & paratestis

Mesothelial tumors

Well differentiated papillary mesothelial tumor



Last author update: 22 December 2023
Last staff update: 22 December 2023

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PubMed Search: Well differentiated papillary mesothelial tumor

See Also: Malignant mesothelioma

Aida Valencia, M.D.
J. Cody Craig, M.D.
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Cite this page: Valencia A, Gordetsky JB, Craig JC. Well differentiated papillary mesothelial tumor. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/testismesothelioma.html. Accessed February 22nd, 2024.
Definition / general
  • Well differentiated papillary mesothelial tumor is a neoplasm of uncertain malignant potential that originates from the mesothelium of the pleura, peritoneum or tunica vaginalis
Essential features
  • Mesothelial tumor without significant cytologic atypia (Ann Diagn Pathol 2019;38:43, Ann Surg Oncol 2019;26:852)
  • Composed entirely of papillary structures and tubules lined by a single layer of bland cuboidal cells with no identifiable invasion
  • Positive staining for mesothelial markers (calretinin, WT1)
  • BAP1 and MTAP immunohistochemical staining should be retained
Terminology
Epidemiology
  • Tumors arising from tunica vaginalis are rare and comprise < 1% of mesotheliomas (Mod Pathol 2022;35:1327)
  • Age range is 18 - 70 years
Sites
Pathophysiology
  • Unknown
Etiology
  • Unknown
Clinical features
Diagnosis
  • Histologic diagnosis is required
Radiology description
Prognostic factors
  • This tumor is likely benign in nature but is often morphologically identical to mesothelioma in situ, which may explain the rare reports of recurrence as mesothelioma (Am J Surg Pathol 2023;47:611)
Case reports
Treatment
  • Complete excision
Gross description
  • Peritoneal surfaces with small nodules or papillary excrescences, single to innumerable (Mod Pathol 2022;35:1327)
Frozen section description
  • These tumors are often found incidentally during other procedures, prompting frozen section evaluation
  • Expanded papillary structures with edema or myxoid change, lined by a single layer of bland mesothelial cells
  • Invasion should be excluded
  • Reference: Int J Surg Pathol 2021;29:844
Microscopic (histologic) description
  • Papillary structures with expansile cores showing myxoid features
  • Papillary structures are lined by a bland simple layer of flattened to cuboidal cells
  • Fibrovascular cores generally lack inflammation and psammoma bodies
  • Occasional gland-like or tubulocystic areas may be seen
  • There should be a lack of stromal invasion, significant atypia and mitotic activity (Mod Pathol 2022;35:1327)
Microscopic (histologic) images

Contributed by Aida Valencia, M.D. and Jennifer Gordetsky, M.D.
Numerous fibrovascular cores Numerous fibrovascular cores

Numerous fibrovascular cores

Background macrophages

Background macrophages

Broad papillae Broad papillae

Broad papillae


Papillae with myxoid stroma Papillae with myxoid stroma

Papillae with myxoid stroma

Nonpsammomatous calcifications Nonpsammomatous calcifications

Nonpsammomatous calcifications

Calretinin

Calretinin


WT1 immunostain WT1 immunostain

WT1

D2-40

D2-40

Malignant mesothelioma Mesothelioma

Mesothelioma

Virtual slides

Images hosted on other servers:

Diagnostic features

Retained BAP1 expression

Retained MTAP expression

Cytology description
  • Clusters of uniform mesothelial cells without atypia
Positive stains
Negative stains
Molecular / cytogenetics description
Molecular / cytogenetics images

Images hosted on other servers:

TRAF7 and CDC42 somatic mutations

Sample pathology report
  • Scrotum, hydrocelectomy:
    • Well differentiated mesothelial tumor
Differential diagnosis
Board review style question #1
Well differentiated papillary mesothelial tumor Well differentiated papillary mesothelial tumor Well differentiated papillary mesothelial tumor


The following tumor is incidentally found on the peritoneum during a cholecystectomy. This tumor should show which immunophenotype?

  1. BAP1 loss, calretinin+, WT1+
  2. BAP1 retained, MTAP retained, calretinin+, WT1+
  3. Cytokeratin 5/6-, D2-40+
  4. p53 null, AE1 / AE3+, PAX8+
Board review style answer #1
B. BAP1 retained, MTAP retained, calretinin+, WT1+. These images show a well differentiated mesothelial tumor, which should have retained BAP1 and MTAP. Mesothelial markers will be positive. Answer A is incorrect because while a well differentiated mesothelial tumor will express mesothelial markers, BAP1 should be retained. Answer C is incorrect because mesothelial tumors will express both CK5/6 and D2-40. Answer D is incorrect because this immunophenotype is more in keeping with an ovarian type serous tumor.

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Reference: Well differentiated papillary mesothelial tumor
Board review style question #2
Well differentiated papillary mesothelial tumor Well differentiated papillary mesothelial tumor Well differentiated papillary mesothelial tumor Well differentiated papillary mesothelial tumor


A patient presents with scrotal swelling that has been persistent over the last few years. An orchiectomy is done. Which of the following immunophenotypes would favor a mesothelioma over a well differentiated papillary mesothelial tumor?

  1. BAP1 loss, calretinin+, WT1+
  2. BAP1 retained, MTAP retained, calretinin+, WT1+
  3. Cytokeratin 5/6-, D2-40+
  4. p53 null, AE1 / AE3+, PAX8+
Board review style answer #2
A. BAP1 loss, calretinin+, WT1+. These images show a tumor with significant cytologic atypia and invasion into the testis. This is a mesothelioma of the tunica vaginalis. Mesotheliomas will express mesothelial markers and have BAP1 loss. Answer B is incorrect because this immunophenotype is in keeping with a benign mesothelial lesion. Answer C is incorrect because mesothelial tumors will express both CK5/6 and D2-40. Answer D is incorrect because this immunophenotype is more in keeping with an ovarian type serous tumor.

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Reference: Well differentiated papillary mesothelial tumor
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