Utility of BRCA1-associated protein 1 immunoperoxidase stain to differentiate benign versus malignant mesothelial proliferations in cytologic specimens

Background: Loss of BAP1 has recently been described as a highly specific marker for distinguishing malignant mesotheliomas (MM) from benign mesothelial proliferations (BMP). The aim of this study was to evaluate the utility of BAP1 in cytospin and cell block (CB) preparations. Methods: We searched the database at our institution for cases of MM (n = 21) and BMP (n = 11). A Papanicolaou stained cytospin preparation and CB section from each case were selected for BAP1 staining. The results were scored as nuclear BAP1 staining, absent or present. Positive staining in non-neoplastic cells was noted as internal control. Results: In the study group, MM cells showed a loss of BAP1 staining in 18/21(86%) cytospin and 19/21(90%) CB preparations. All cytospins showed appropriate positive nuclear staining in the “internal control” inflammatory cells. However, only 17/21(81%) of CB sections showed good internal control with positive nuclear staining in inflammatory cells. The intensity of BAP1 nuclear staining varied across cases. Overall, the intensity of staining was higher in cytospins, providing sharper contrast between the malignant cells with loss of stain and the benign inflammatory cells with retained stain in cases of MM. Sensitivity/specificity for cytospins when compared with CB was 86/100 and 88/100%, respectively. Conclusions: Cytospin preparations have comparable sensitivity and specificity to CB with greater intensity and better contrast between negative and positive nuclei making the interpretation easier. We advise to interpret BAP1 IPOX stain results with caution with careful attention to staining of background nonmalignant “internal control” cells. Diagn. Cytopathol. 2017;45:312–319.