Answer: Malignant mesothelioma

Histology: Behind the hydrocele sac are multiple papillary excrescences. In areas, these papillae are lined by a single cuboidal layer of bland cells. Other areas show a more solid growth pattern with nuclear enlargement and moderate nuclear pleomorphism. Rare mitotic figures are identified, including one that was atypical.

Discussion: Although one may see florid reactive mesothelial proliferation in hydrocele sacs, they are characterized by sheets of reactive mesothelial cells with focal tubular proliferations in the underlying stroma, often associated with marked inflammation. In the current case, there is no inflammation to invoke a reactive process and the presence of nodular papillary lesions is inconsistent with a reactive mesothelial proliferation. The key differential diagnosis in this case rests between well-differentiated papillary mesothelioma and a well-differentiated malignant mesothelioma. Well-differentiated papillary mesotheliomas typically occur in women in the pelvic peritoneum. They are typically covered by a single layer of flattened cuboidal mesothelial cells, bland cytology and lack mitotic figures. In the current case, given the multifocality of these lesions, along with areas of solid growth and moderate nuclear pleomorphism and scattered mitotic figures, one cannot be assured that this lesion will have a benign clinical course, which is the implication of the diagnosis of well-differentiated papillary mesothelioma. On the other hand, this lesion is not overtly high-grade, and there needs to be some designation that this lesion may still have relatively good behavior. Consequently, we would regard this as a well-differentiated malignant mesothelioma.