Well differentiated papillary mesothelial tumor: a new name and new problems.

Well-differentiated papillary mesothelial tumor (WDPMT, formerly called well-differentiated papillary mesothelioma) is a morphologically distinctive lesion composed of expansile papillae with a myxoid core covered by a single layer of generally bland mesothelial cells. Whether some WDPMT are precursors of invasive mesothelioma is uncertain, and this question is confounded by shallow biopsies of ordinary diffuse mesotheliomas that have superficial areas resembling WDPMT as well as by misinterpretation of some cases of mesothelioma in situ. Genetic analyses on a very small number of published cases of peritoneal WDPMT have shown a variety of mutations/copy number losses that do not overlap at all with those that are found recurrently in invasive mesotheliomas. The newly described entity of mesothelioma in situ usually appears as a single layer of mesothelial cells that have lost BAP1 by immunostaining, but sometimes is papillary and produces a morphologic mimic of WDPMT. We propose that, at least in the peritoneal cavity where most WDPMT occur, there are two morphologically identical but functionally distinct lesions: one is true WDPMT, a process that is probably benign, and the other is papillary mesothelioma in situ with the configuration of WDPMT. For that reason immunostaining for BAP1, and if necessary MTAP or CDKN2A FISH, should always be performed on cases with the appearance of WDPMT. It is possible, but speculative, that the small number of reports in the literature which describe invasive mesothelioma arising from WDMPT are actually describing invasive mesothelioma arising from mesothelioma in situ that looks like WDPMT.