A Case of Primary Malignant Lymphoma of the Prostate Gland Presenting as Right Lower Back Pain and Dysuria.

A 73-year-old man presented with right lower back pain and dysuria. Right hydronephrosis and a large pelvic large mass were seen on computed tomography (CT). Although his prostate-specific antigen (PSA) was 0.5 ng/mL, an irregularly enlarged, stony, hard prostate was palpable on digital rectal examination. A prostate tumor was suspected, and a transrectal prostate biopsy and right transurethral ureteral stent placement were performed. Histological and immunohistochemical studies revealed diffuse large B-cell lymphoma. Positron emission tomography-computed tomography showed abnormal uptake in the stomach, cecum, right obturator lymph nodes, para-aortic lymph nodes, and dorsal left kidney. No abnormal findings were seen on bone marrow histology. Clinical stage IVA was confirmed according to Ann Arbor criteria. The patient achieved a complete response after 8 cycles of combination chemotherapy with rituximab, pirarubicin, cyclophosphamide, vincristine, and prednisolone.