Context: The separation of benign from malignant mesothelial proliferations is crucial to patient care but is frequently morphologically difficult.
Objective: To briefly review adjunctive tests claimed to be useful in this setting and to examine in detail 2 new tests: p16 fluorescence in situ hybridization (FISH) and BRCA1-associated protein 1 (BAP1) immunohistochemistry.
Design: Literature review with emphasis on p16 FISH and BAP1 immunohistochemistry.
Results: Glucose transporter-1, p53, insulin-like growth factor 2 messenger RNA-binding protein 3 (IMP-3), desmin, and epithelial membrane antigen have all been claimed to mark either benign or malignant mesothelial processes, but in practice they at best provide statistical differences in large series of cases, without being useful in an individual case. Homozygous deletion of p16 by FISH or loss of BAP1 has only been reported in malignant mesotheliomas and not in benign mesothelial proliferations. BAP1 appears to be lost more frequently in epithelial than mixed or sarcomatous mesotheliomas. Homozygous deletion of p16 by FISH is seen in pleural epithelial, mixed, and sarcomatous mesotheliomas, but it is much less frequent in peritoneal mesothelioma. The major drawback to both these tests is limited sensitivity; moreover, failure to find p16 deletion or BAP1 loss does not make a mesothelial process benign.
Conclusions: In the context of a mesothelial proliferation, the finding of homozygous deletion of p16 by FISH or loss of BAP1 by immunohistochemistry is, thus far, 100% specific for malignant mesothelioma. The limited sensitivity of each test may be improved to some extent by running both tests.