[Clinicopathological characteristics of well-differentiated papillary mesothelial tumor]

Objective: To investigate the clinicopathological characteristics of well-differentiated papillary mesothelial tumor (WDPMT). Methods: Sixteen cases of resected WDPMTs diagnosed at the Affiliated Hospital of Qingdao University, Qingdao, China from 2017 to 2024 were collected and the clinicopathological features were retrospectively analyzed. Results: There were 7 males amd 9 females, with a mean age of 53.8±14.8 years (range, 25-83 years). Tumor size ranged from 3 to 12 mm in maximum diameter. Of the 16 cases, 15 involved the peritoneum and 1 involved the pleura, one of which occurred on the surface of ovary. All cases were incidentally identified during unrelated surgical procedures. Histologically, tumors exhibited arborizing papillary growth patterns and frequently displayed hierarchically branching papilla. Tumor cells showed cuboidal to flattened cell morphology with minimal nuclear atypia. Mitotic figures were not noted in all cases. Entrapped gland-like tumor cell clusters were found in the stroma of tumor papilla in 1 of the 16 cases. Immunohistochemically, the tumor cells expressed mesothelial markers (Calretinin, D2-40, and CK5/6) in all cases, and BAP1 and MTAP were immunoreactive in all tested cases. Fluorescence in situ hybridization revealed no CDKN2A deletions. Conclusions: WDPMT predominantly occurs in the peritoneum and typically demonstrates indolent biological behaviors. It often shows overlapping features with mesothelioma in situ and epithelioid mesothelioma. The hierarchical branching papillae is its diagnostic hallmark, while routine immunohistochemical evaluation of BAP1 and MTAP is also recommended for differential diagnosis of these tumors.