A 70-year-old Asian male patient presented with a 2-year history of intermittent dysuria. He had symptoms of thinner urine flow, shorter range, prolonged urination, and incomplete urination, accompanied by urgency and frequent urination. Initially, because the symptoms were still mild and did not significantly affect daily life, the patient did not pay attention and was not diagnosed and treated systematically. Since May 2019, the above symptoms of the patient were significantly worse than before, and he was admitted to the urology department of our hospital for further treatment. There was no antecedent history of illness or trauma and no family history of malignancies. Physical examination did not reveal any abnormal finding except of diffusely enlarged prostate during rectal examination. Pelvic magnetic resonance imaging (MRI) showed that the peripheral zone of the prostate was occupied, which was considered because prostate cancer (or sarcoma) invades the bilateral seminal vesicle glands, the posterior wall of the bladder, and the anterior wall of the rectum with pelvic lymph node metastasis. However, his serum prostate-specific antigen (PSA) was 0.404 ng/ml, which was within the normal range. A prostate biopsy was performed, identifying a non-Hodgkin lymphoma whose immunohistochemical test was positive for antibodies CD20, MUM-1, Bcl-6, CD79a, and CD5 and negative for CD3, CD30, CyclinD1, PSA, P504s, NKX3.1, P63, 34βE12, CgA, Syn, CD56, and CK-pan. Also, Ki-67 staining was 70% positive, showing a high proliferation and invasiveness of the tumor. Gene rearrangement suggested that both immunoglobulin heavy chain (IgH) and T-cell receptor gamma (TCRG) were positive. These testing results supported the diagnosis of diffuse large B-cell lymphoma. At this time, the patient began to have nausea, vomiting, intermittent fever, fatigue, night sweats, difficulty in defecation, and obvious edema of both lower limbs, scrotum, and abdominal wall. The patient was transferred to our department since July 2019 for specialized therapy. Neck, chest, and full-abdomen enhanced computed tomography (CT) revealed an anterior mediastinum soft tissue shadow, and multiple lymph nodes in the pelvic cavity, retroperitoneal area, and bilateral inguinal areas, which were all considered to be prostate lymphoma with infiltration of the bladder, seminal vesicles, rectum, and mediastinum. Bone marrow biopsy did not indicate lymphoma infiltration. The patient’s complete blood count results were generally normal, and his lactate dehydrogenase (LDH) test result was 326 U/L, which was above the normal range. The patient received eight courses of chemotherapy with the R-CHOP regimen (including rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone). Post-chemotherapy CT scans showed complete remission. Following chemotherapy, the patient received intensity-modulated radiotherapy (IMRT) of the prostate with a clinical target volume (CTV) dose of 40 Gy/20 f, including the prostate region. He finished radiation therapy in February 2020 without further treatment, and his condition has stabilized without progression on recent follow-up, with no further symptoms of urinary tract obstruction.