After radiotherapy (external beam radiotherapy and/or brachytherapy), the PSA ought to drop to low levels and remain low. The time taken for this fall is different from that following surgery, since the prostate gland remains in the body and PSA is produced by both normal and cancer cells. The lowest PSA reading (called the nadir) may not occur for up to 18 months following radiotherapy. Ideally the PSA ought to fall to below 1.0 ng/mL; at this level, the outlook for cancer control is very good.

Up to one-third of men who have had brachytherapy will have a temporary rise in PSA during the first 12–24 months. After this, the PSA will resume falling. This is referred to as PSA bounce. The cause of PSA bounce is not well understood. It can raise concern about cancer growth for patient and doctor, but patience, with continued 3-monthly PSA testing, is usually the best strategy.

The frequency of measuring the PSA after radiotherapy is somewhat controversial. Many specialists would recommend once every 3–6 months for the first 2 years, 6 monthly for 2–5 years and annually thereafter.