A Rare Case of Peritoneal Well-Differentiated Papillary Mesothelioma Coincidentally Associated with Spermatic Cord Mesothelial Hyperplasia

Background: We report a case of incidental association between Well-Differentiated Papillary Mesothelial Tumor (WDPMT) of the peritoneum and Nodular Histiocytic/Mesothelial Hyperplasia (NHMH) of the spermatic cord.

Case Presentation: The male patient was in his 40s with no history of asbestos exposure. A computed tomography scan of a gradually enlarging inguinal hernia revealed seminal hydrocele and ascites. Laparoscopy revealed multiple small nodules in the peritoneum. Histologically, the seminal hydrocele contained a nodule within the hernia sac. The nodule was mainly composed of histiocytes positive for CD68 (Cluster of Differentiation 68), with a few scattered reactive mesothelial cells positive for calretinin and epidermal growth factor-like domain 1 (HEG1). The spermatic cord lesion was diagnosed as NHMH. The resected peritoneal nodule had an edematous stroma with a highly branching papillary pattern, and the surface was covered with a single layer of non-dysplastic mesothelial cells positive for calretinin and HEG1. No nuclear deletion of BRCA1-Associated Protein 1 (BAP1), Methyl-Thioadenosine Phosphorylase (MTAP), or Merlin/Neurofibromatosis 2 (NF2) was found in the superficial mesothelial cells, and the diagnosis was WDPMT.

 

Conclusion: In some cases, WDPMT may not be differentiated from Mesothelioma In Situ (MIS), or from diffuse or localized mesothelioma. In cases of MIS as well as diffuse or localized mesothelioma with a conspicuous papillary pattern, these tumors are difficult to differentiate from WDPMT when only the superficial components are obtained by biopsy, particularly owing to the lack of confirmation of invasion. In such cases, the findings of nuclear deletion of BAP1 (BRCA1-associated protein 1), MTAP, and Merlin/NF2 using immunohistochemistry and homozygous deletion of p16 cyclin-dependent kinase 1 inhibitor 2A using fluorescent in situ hybridization help to exclude the possibility of WDPMT or diagnose MIS, and diffuse or localized mesothelioma. Given the rarity of WDPMT, molecular studies are still underway, and further research is needed.