BAP-1 in the diagnosis of malignant pleural mesothelioma

The diagnosis of malignant pleural mesothelioma (MPM) can be challenging for clinicians and pathologists.

This study included all patients diagnosed with pleural mesothelioma between October 2013 and April 2024. Clinical data, history of asbestos exposure, pleural fluid analysis, chest-X-ray, thoracic computed tomography and positron emission tomography findings, video-assisted thoracic surgery report, pleural fluid cytology and pleural biopsy results were included. Loss of BRCA1-associated protein 1 (BAP1) nuclear staining was examined in cytologies and biopsies.

Forty-one patients were diagnosed with pleural mesothelioma, 35 epithelioid, 5 sarcomatoid and 1 biphasic. The diagnosis was established after the first sample in 30 patients, while a second one was necessary in 11 patients. BAP1 loss in pleural cytology and histology was not always consistent. Sixteen patients showed loss of BAP1 expression in cytology. Among these, 10 patients also presented BAP1 loss in pleural histology, whereas 5 patients with BAP1 loss in histology did not show BAP1 loss in cytology. Additionally, two patients initially diagnosed with idiopathic pleural effusion and 3 patients diagnosed with benign pleural effusion due to asbestos showed BAP1 loss and were later diagnosed with MPM during follow-up.

MPM remains difficult to diagnose. Performing BAP1 immunohistochemistry in patients with an undiagnosed pleural effusion or a history of asbestos exposure can aid in identifying those with mesothelioma.