Prostate-specific antigen, or PSA, levels fall immediately after radiation treatment, according to the University of California San Francisco Medical Center. However, PSA levels usually take between two and three years to reach their lowest point after radiation.
Radiation therapy does not kill all of the cells in the prostate glands, explains the American Cancer Society. The remaining cells continue creating PSA. PSA levels typically drop gradually following radiation therapy, but may fluctuate. Minor fluctuations are usually not cause for alarm, but a sharp increase may warrant closer monitoring. Additionally, consistently rising PSA levels may indicate that the cancer is still active.