Rapidly Progressing Prostate Cancer With Low Prostate-Specific Antigen and Gleason Score 5+5: A Case Report.

An 87-year-old man presented with worsening urinary dysfunction with urinary retention. The prostate-specific antigen was 1.23 ng/mL. Prostate biopsy confirmed adenocarcinoma with a Gleason score of 5+5; the final diagnosis was T4N1M0. Androgen deprivation therapy and immediate whole-pelvic radiotherapy (80 Gy) were administered. Nadir prostate-specific antigen of 0.40 ng/mL two months later increased to 35.92 ng/mL as the disease metastasized to the corpus cavernosum penis, causing malignant priapism. The best supportive care was provided, and the patient died 11 months later. High-grade prostate cancers with low prostate-specific antigen levels are rare but highly aggressive. Androgen deprivation therapy and radiotherapy alone may be insufficient in such cases, highlighting the need for more effective treatment strategies.