New developments in mesothelial pathology

Area of Science:

• Pathology
• Oncology
• Immunohistochemistry

Background:

• Mesothelial pathology presents diagnostic challenges, particularly distinguishing in situ lesions from benign mimics and differentiating well-differentiated papillary mesothelial tumor (WDPMT) from invasive mesothelioma.
• Loss of BAP1 and/or MTAP by immunohistochemistry defines mesothelioma in situ, but benign reactions can mimic this finding.
• The behavior of WDPMT remains debated, with some cases resembling mesothelioma in situ.

Purpose of the Study:

• To review current understanding and problem areas in mesothelial pathology.
• To clarify the concept and diagnosis of mesothelioma in situ and WDPMT.
• To introduce novel immunohistochemical markers for accurate mesothelial tumor diagnosis.

Main Methods:

• Review of existing literature and diagnostic criteria for mesothelial lesions.
• Analysis of immunohistochemical markers, including BAP1, MTAP, HEG1, and claudin-4.
• Discussion of differential diagnoses between benign and malignant mesothelial proliferations.

Main Results:

• Mesothelioma in situ is characterized by bland mesothelial cells lacking BAP1/MTAP, with benign effusions as a potential mimic.
• WDPMT diagnosis requires careful evaluation, with BAP1 staining recommended to differentiate from mesothelioma in situ.
• HEG1 and claudin-4 demonstrate high sensitivity and specificity for distinguishing epithelioid mesothelioma from non-small cell lung cancer.

Conclusions:

• Mesothelioma in situ and WDPMT are associated with an increased risk of invasive mesothelioma development.
• Immunohistochemistry, particularly BAP1, is crucial for diagnosing mesothelioma in situ and WDPMT.
• HEG1 and claudin-4 are valuable tools for differentiating epithelioid mesothelioma from other malignancies.