Mesothelioma In Situ Mimicking Well-differentiated Papillary Mesothelial Tumor.

We have previously hypothesized that well-differentiated papillary mesothelial tumor (WDPMT) consists of 2 morphologically identical lesions, one of which is true WDPMT, while the other is a form of mesothelioma in situ. Here, we report 8 examples of the latter phenomenon, 3 with pleural disease (2 men/1 woman, ages 66 to 78 y); and 5 with peritoneal disease (all women, ages 31 to 81 y). At presentation the pleural cases all had effusions but no evidence of pleural tumor on imaging. Four of the 5 peritoneal cases had ascites as the initial finding and all 4 had nodular lesions that by imaging and/or direct inspection were thought to represent a diffuse peritoneal malignancy. The fifth peritoneal case presented with an umbilical mass. Microscopically, the pleural and peritoneal lesions looked like diffuse WDPMT, but all had lost BAP1. Occasional microscopic foci of superficial invasion were present in 3/3 pleural cases, while single nodules of invasive mesothelioma and/or occasional foci of superficial microscopic invasion were found in all of the peritoneal cases. The pleural tumor patients developed what clinically appeared to be invasive mesothelioma at 45, 69, and 94 months. Four/five peritoneal tumor patients underwent cytoreductive surgery and heated intraperitoneal chemotherapy. Three with follow-up data are alive without recurrence at 6, 24, and 36 months; 1 patient refused treatment but is alive at 24 months. We conclude that mesothelioma in situ morphologically mimicking WDPMT is strongly associated with the synchronous or metachronous development of invasive mesothelioma, but that these lesions appear to progress very slowly.