Well-differentiated papillary mesothelioma of the peritoneum: a borderline mesothelioma. Report of two cases and review of literature.

Well-differentiated papillary mesothelioma (WDPM) is considered to be a distinct subtype of peritoneal mesothelioma. Although the WDPM is usually qualified as benign, the natural history of this lesion has not been clearly established. This report relates to two WDPMs which were found incidentally. In a 51-year-old man the WDPM developed over a period of 5 years into a typically malignant diffuse mesothelioma. Thus, although the WDPM morphologically lacks signs of malignancy, it should be regarded as a borderline mesothelioma.