Utility of Methylthioadenosine Phosphorylase Compared With BAP1 Immunohistochemistry, and CDKN2A and NF2 Fluorescence In Situ Hybridization in Separating Reactive Mesothelial Proliferations From Epithelioid Malignant Mesotheliomas

Context.— The separation of reactive from malignant mesothelial proliferations is often a difficult morphologic problem. There is contradictory information in the literature on whether methylthioadenosine phosphorylase (MTAP) immunohistochemistry can be used for this purpose. Objective.— To determine the utility of MTAP immunohistochemistry in distinguishing reactive from malignant mesothelial proliferations. Design.— We stained a tissue microarray containing 20 epithelioid malignant mesotheliomas and 17 reactive mesothelial proliferations. For the mesotheliomas, comparisons were made between MTAP staining and BRCA-associated nuclear protein 1 (BAP1) immunohistochemistry, cyclin-dependent kinase inhibitor 2A (CDKN2A) fluorescence in situ hybridization, and neurofibromin 2 (NF2) fluorescence in situ hybridization, which are established techniques for making this separation. Results.— Loss of MTAP was seen in 0 of 17 reactive mesothelial proliferations and 13/20 (65%) malignant mesotheliomas. Almost all cas...