Changes in serum PSA levels of patients who underwent salvage radiotherapy (SRT) for recurrent disease after radical prostatectomy. Patients were evaluated by PSA measurement at regular intervals after completing SRT. Patients who showed a decline to PSA < 0.2 ng/mL after SRT and maintained PSA < 0.2 ng/mL during the follow-up were categorized as the SRT success group (left). Patients who showed a decline to PSA < 0.2 ng/mL after SRT and thereafter experienced a rise in PSA at two consecutive measurement points with the last PSA ≥ 0.2 ng/mL (middle) were categorized as the recurrence group. If the PSA continued to rise without a decline to PSA < 0.2 ng/mL after SRT, patients were categorized as the nonresponse group (right). The recurrence group and the nonresponse group were defined as the SRT failure group.