Answer: A

Histological Description: At the interface of more dense fibrous and more reactive inflammatory fibrous tissue there is a linear proliferation of tubules with focal papillary formation. Cytologically, the tubules are lined by bland cuboidal epithelium.

Discussion. Hydroceles which are fluid-filled mesothelial-lined sacs surrounding the testis that in adults usually does not communicate with the peritoneal cavity. The fluid accumulates as a reaction to injury, most often as a reaction to infection (i.e. epididymitis). In this setting, the hydrocele sac may have an associated florid reactive mesothelial proliferation, mimicking malignant mesothelioma. Further compounding the diagnostic difficulty, is that hydroceles can also arise as a reaction to malignancy in this site, such as malignant mesothelioma. The key to recognizing that the process is not malignant is that the reactive proliferation remains confined to a sharply demarcated zone immediately underlying the luminal surface. Proliferating mesothelial cells typically form lines that parallel the surface of hydrocele and fail to penetrate beyond the associated superficial zone of inflammation and fibrosis. Tubules do not invade adipose tissue. Architecturally, simple papillary structures, tubules and nests can be present, but solid areas and broad arborizing complex papillary structures with hyalinized fibrous cores are not identified. Reactive cells maintain abundant cytoplasm and may contain enlarged vesicular nuclei and brisk mitotic activity in inflamed areas. Cytologically, malignant mesothelioma in this region can also have relatively uniform bland cuboidal cells with only modest amount of eosinophilic cytoplasm, although occasionally, frank anaplasia may be demonstrated. Architecturally, malignant mesothelioma has broad arborizing complex papillary and tubular structures and nests with occasional solid areas. Although at times predominantly involving surface as exophytic papillary growth, at least focally, a haphazard infiltrative tubular or nested component exists. Malignant mesotheliomas often invades adipose tissue. In general, immunohistochemistry is often not helpful in the differential diagnosis, although malignant mesothelioma can show loss of MTAP and BAP1 in 50% of malignant mesotheliomas and not reactive mesothelial proliferations.