Extranodal Rosai-Dorfman disease of the kidney and coexistent poorly differentiated prostatic adenocarcinoma.

We present a case of a patient who was initially diagnosed with poorly differentiated prostatic adenocarcinoma on prostate needle core biopsy. Upon staging workup, a computed tomographic scan showed a 9-cm left renal mass involving mainly the pelvicalyceal system. Positron emission tomographic scan showed increased uptake in para-aortic, paracaval, and retrocaval lymph nodes suspicious for metastatic disease. Left nephrectomy revealed involvement with extranodal Rosai-Dorfman disease. Lymphadenectomy revealed metastatic prostatic adenocarcinoma; however, the lymph nodes did not show evidence of Rosai-Dorfman disease. Isolated involvement of the kidney by Rosai-Dorfman disease is very rare. The combination of prostatic adenocarcinoma and isolated extranodal Rosai-Dorfman disease of the kidney makes this case unique.