Original ArticlesPAX8 Expression in a Subset of Malignant Peritoneal Mesotheliomas and Benign Mesothelium has Diagnostic Implications in the Differential Diagnosis of Ovarian Serous CarcinomaChapel, David B. MD*; Husain, Aliya N. MD*; Krausz, Thomas MD*; McGregor, Stephanie M. MD, PhD† Author Information *Department of Pathology, The University of Chicago, Chicago, IL †Department of Pathology and Laboratory Medicine, Wisconsin Institute for Medical Research, The University of Wisconsin-Madison, Madison, WI Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Stephanie M. McGregor, MD, PhD, Department of Pathology and Laboratory Medicine, Wisconsin Institute for Medical Research, The University of Wisconsin-Madison, 1111 Highland Avenue, B1793 WIMR, Madison, WI 53792 (e-mail: [email protected]). The American Journal of Surgical Pathology: December 2017 - Volume 41 - Issue 12 - p 1675-1682 doi: 10.1097/PAS.0000000000000935 Buy Metrics Abstract Distinguishing malignant peritoneal mesothelioma (MPM) from serous carcinoma involving the peritoneum remains a diagnostic challenge, particularly in small biopsy and cytology specimens. In this distinction, PAX8 expression has been regarded as a specific marker of serous carcinoma. In addition, BAP1 loss is reportedly specific to MPM, in the distinction from both benign mesothelial lesions and ovarian serous tumors (OSTs). Using immunohistochemistry, we examined PAX8 and BAP1 expression in 27 MPMs, 25 cases of benign mesothelium, and 45 OSTs. Five MPMs were PAX8+ (5/27, 18%), while 8 cases of benign mesothelium expressed PAX8 (8/25, 32%). PAX8 expression in mesothelium was significantly more common in women than in men (P=0.01). Sixteen MPMs exhibited BAP1 loss (16/25, 64%), while BAP1 was retained in all benign mesothelium and all OSTs. All cases of PAX8+ mesothelium were negative for expression of estrogen receptor. These data show that PAX8 is expressed in both benign and malignant mesothelium, and that BAP1 loss is highly specific for MPM, in the differential with both benign mesothelial proliferations and OTSs. These results also have implications for primary diagnosis and for pathologic staging of OST. Caution should be applied when PAX8 expression is used to distinguish mesothelial and serous proliferations, and BAP1 loss may be confirmatory in cases where mesothelioma is favored. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.