Incidental finding of multiple well‐differentiated papillary mesotheliomas in peritoneum

We present a case of multiple well‐differentiated papillary mesotheliomas ( WDPM ) in the peritoneum found incidentally in a 63‐year‐old man with urothelial carcinoma of the bladder. When multiple tumors are seen, malignant mesothelioma should be excluded by histopathological examination as this may have a similar focal appearance to WDPM . True stromal invasion is by far the most reliable criterion of mesothelial malignancy. In doubtful cases, a conservative diagnostic approach has been recommended. Compared to malignant mesotheliomas, WDPM s are rare and have a relatively indolent clinical course with a good prognosis. Great care is needed when diagnosing mesothelial proliferations, given the crucial nature of a benign vs malignant diagnosis. No standardized treatment has yet been established.