Aberrant Pax-8 expression in well-differentiated papillary mesothelioma and malignant mesothelioma of the peritoneum: a clinicopathologic study

Serous ovarian neoplasms can overlap morphologically with peritoneal mesothelial proliferations, including well-differentiated papillary mesothelioma (WDPM) and malignant epithelioid mesothelioma (MM). Accurate histologic classification of these neoplasms is important for clinical management. The Pax-8 protein is commonly used for differentiating peritoneal MM from serous carcinoma, but the diagnostic value of Pax-8 for distinguishing WDPM from borderline or low-grade serous tumors is unknown. We used immunohistochemistry staining to assess Pax-8 expression in 33 WDPMs, 34 peritoneal MMs, 48 pleural MMs, 11 adenomatoid tumors, 5 peritoneal inclusion cysts, and 51 benign/reactive mesothelium specimens. Staining was noted in 20 WDPMs (61%), with 17 showing strong and diffuse nuclear staining and 3 patchy/focal staining. Calretinin was expressed in 33 cases (100%), whereas focal BerEP4 staining was noted in 2 of 29 cases (7%). In contrast, 4 peritoneal MM (12%) were Pax-8 positive (3 diffuse and 1 focal staining). All adenomatoid tumors and peritoneal inclusion cysts were negative for Pax-8. Of the 48 pleural MM cases, 2 (4%) showed focal weak to moderate nuclear labeling for Pax-8, and 2 cases (4%) of reactive mesothelium demonstrated focal and scattered Pax-8 staining. Pax-8 appears to be a useful marker for distinguishing MM from gynecologic malignancies but is not reliable for distinguishing WDPM from borderline or low-grade gynecologic lesions.