Answer: Rosai-Dorfman disease

Histology: The background testis is severely atrophic. In the interstitium is a polymorphous cellular infiltrate composed of lymphocytes, plasma cells, and histiocytes with hemosiderin deposition. Some of the histiocytes have engulfed lymphocytes and other inflammatory elements.

Discussion: The findings in this case are classic for Rosai Dorfman disease. The finding of histiocytes with engulfed inflammatory cells (emperipolesis) is pathognomonic. Idiopathic granulomatous orchitis is an autoimmune disease where the granulomatous infiltrate is confined within the seminiferous tubules. Sarcoidosis would have well-formed granulomas. Infections would similarly have well formed granulomas, possibly with necrosis, typically first involving the epididymis before eventually involving the testis. It is necessary to rule out seminoma when you see an ill-defined granulomatous infiltrate in the testis. In this case, the finding of emperipolesis and lack of atypical seminoma cells is diagnostic of Rosai Dorfman disease. This disease is very rare in the testis and most cases Most patients are middle aged. Bilateral disease is common and it may involve other sites, most commonly lymph nodes.