Context.—

Mesothelioma is an uncommon tumor that can be difficult to diagnose.

Objective.—

To provide updated, practical guidelines for the pathologic diagnosis of mesothelioma.

Data Sources.—

Pathologists involved in the International Mesothelioma Interest Group and others with expertise in mesothelioma contributed to this update. Reference material includes peer-reviewed publications and textbooks.

Conclusions.—

There was consensus opinion regarding guidelines for (1) histomorphologic diagnosis of mesothelial tumors, including distinction of epithelioid, biphasic, and sarcomatoid mesothelioma; recognition of morphologic variants and patterns; and recognition of common morphologic pitfalls; (2) molecular pathogenesis of mesothelioma; (3) application of immunohistochemical markers to establish mesothelial lineage and distinguish mesothelioma from common morphologic differentials; (4) application of ancillary studies to distinguish benign from malignant mesothelial proliferations, including BAP1 and MTAP immunostains; novel immunomarkers such as Merlin and p53; fluorescence in situ hybridization (FISH) for homozygous deletion of CDKN2A; and novel molecular assays; (5) practical recommendations for routine reporting of mesothelioma, including grading epithelioid mesothelioma and other prognostic parameters; (6) diagnosis of mesothelioma in situ; (7) cytologic diagnosis of mesothelioma, including use of immunostains and molecular assays; and (8) features of nonmalignant peritoneal mesothelial lesions.

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Competing Interests

The authors have no relevant financial interest in the products or companies described in this article.

Author notes

This work was presented at the International Mesothelioma Interest Group meeting; June 27, 2023; Lille, France.

Attanoos provides expert testimony in asbestos litigation for claimants/plaintiffs, defendants, and on a joint basis. Roggli and Beasley consult with attorneys representing plaintiffs and defendants in asbestos litigation.

Supplementary data