Mesothelial Lesions of the Testis

Mesothelial lesions of the testis and paratestis encompass a spectrum of reactive, benign, and malignant processes that may overlap morphologically with one another and with other intra- and extratesticular neoplasms, including Müllerian-type tumors as well as Sertoli cell and yolk sac tumors. Providing an accurate diagnosis has important consequences for patient care, dictating significant changes in treatment modality. While immunohistochemical stains specific to mesothelial cells are helpful in differentiating mesothelial from nonmesothelial lesions, the distinction between benign and malignant processes has only recently been facilitated by molecular testing and immunohistochemical staining for BAP1 and MTAP. Although benign mesothelial lesions are usually easily classified with routine histopathologic evaluation, certain cases of florid reactive mesothelial hyperplasia and infarcted adenomatoid tumors may pose a diagnostic challenge. Well-differentiated papillary mesothelial tumors are bland papillary tumors with indolent behavior and TRAF7 or CDC42 mutations. Architecturally more complex forms within this spectrum exist and have been labelled as having "uncertain malignant potential." Malignant mesothelioma of the tunica vaginalis is an uncommon but aggressive tumor that may show loss of BAP1 or MTAP immunostaining, supporting a distinct molecular pathogenesis. This review summarizes the clinicopathologic, immunohistochemically, and molecular features of mesothelial lesions of the paratestis, emphasizing their morphologic overlap, diagnostic pitfalls, and evolving framework for classification.