Well-differentiated papillary mesothelioma: A 17-year single institution experience with a series of 75 cases.

We present our experience with 75 cases of well-differentiated papillary mesothelioma (WDPM) that were diagnosed at our institution between 2000 and 2017. The patients included 58 females and 17 males with age ranging from 18 to 69 years (mean, 42 years). Clinically, the vast majority of WDPMs were incidental findings during laparotomy or laparoscopic surgery for a variety of benign or malignant disease. The lesion manifested as either a small solitary nodule or multiple miliary nodules on the peritoneum or serosal surfaces of internal organs. Histologically, 67 cases were consistent with a classical WDPM, of which 6 cases contained microinvasive foci and 1 case had malignant transformation. Eight cases were hybrid tumors with variable combined component of adenomatoid tumor (n = 4), multicystic mesothelioma (n = 2), and both (n = 2). By immunohistochemistry, besides calretinin, D2-40, CK5/6 and WT1, 94% (29/31) of cases also showed immunostaining for PAX8. In comparison, PAX8 staining was only present in 12% (6/50) of epithelioid malignant mesothelioma selected as control cases. Follow-up information available in 46 cases revealed no signs of tumor progression or local recurrence except for the case that showed transformation to a fully malignant mesothelioma after a period of 15 years. Our comprehensive study further expanded the clinical and histopathological spectrum of WDPM. Compared with epithelioid malignant mesothelioma, PAX8 staining is highly sensitive and specific for WDPM (P < 0.001).