LAUSR

The separation of benign from malignant mesothelial cells is often a challenging problem. Some studies have suggested that immunohistochemical staining of CD146 can be used to make this distinction, but there are marked differences in the reported results. Here, we assessed CD146 expression in tissue microarray specimens of 32 epithelioid reactive mesothelial hyperplasias, 17 spindle cell reactive mesothelial proliferations, 43 epithelioid mesotheliomas, and 31 sarcomatoid mesotheliomas. We found that, although the specificity of CD146 for epithelioid mesotheliomas versus reactive epithelial mesothelial proliferations was high (94%), staining intensity and extent was usually low and sensitivity was poor (23%). For sarcomatoid mesotheliomas versus reactive spindle cell mesothelial processes, both measures (33% sensitivity, 76% specificity) were inadequate. Furthermore, strong staining of endothelial cells and fibroblasts often created difficulties in interpretation. In comparison, BAP1 was lost in 21/43 (49%) epithelioid and 9/31 (29%) sarcomatoid mesotheliomas and methylthioadenosine phosphorylase (MTAP) was lost in 9/40 (23%) epithelioid and 7/29 (24%) sarcomatoid mesotheliomas from these TMAs. There was no association between CD146 staining and BAP1 or MTAP retention/loss. We conclude that CD146 staining is probably not useful for separating malignant from benign mesothelial proliferations.