Well-differentiated papillary mesothelioma: a clinicopathological and immunohistochemical study of 18 cases with additional observation

Aims To present our experience with 18 cases of well-differentiated papillary mesothelioma (WDPM), with an emphasis on its relationship to adenomatoid tumour and multicystic mesothelioma. Methods and results Eighteen cases of WDPM were retrieved. Twenty cases of multicystic mesothelioma were selected for comparative study. The WDPM patients comprised 14 females and four males, with age ranging from 18 to 60years (median 37years). The tumour involved the abdominal or pelvic peritoneum (14 cases), the pleura (two cases), and the testicular tunica vaginalis (two cases). The majority of WDPMs were incidental findings during surgery for a wide variety of conditions. Histologically, 13 cases were consistent with classical WDPM. Two cases had a combined component of adenomatoid tumour, and three cases contained coexisting areas of multicystic mesothelioma. Among the multicystic mesothelioma cases, four had adenomatoid tumour-like foci present in the stroma. Immunohistochemically, all cases of WDPM and multicystic mesothelioma, as well as the coexisting combined components, were positive for mesothelial markers, with a low Ki67 index. This study also showed that WDPM was negative for epithelial membrane antigen and desmin. Conclusions The simultaneous occurrence of WDPM, adenomatoid tumour and multicystic mesothelioma in some cases suggested histogenetic relationships among these three less aggressive mesotheliomas.