PURPOSE: To develop and compare normal tissue complication probability (NTCP) models for recurrent brain metastases (BMs) treated with repeat single-fraction stereotactic radiosurgery (SRS), considering time-dependent discounted prior dose. METHODS AND MATERIALS: We developed three NTCP models (M0, M1-retreat, and M1-combo models) of BMs treated with GammaKnife-based SRS. The maximum dose is 0.2 cc (D RESULTS: All 3 models fitted the data well (χ CONCLUSIONS: The model-fitted parameters predicted that recurrent BMs would have a lower threshold dose tolerance and a more gradual dose response to the second SRS than nonrecurrent BMs. This gradual dose-response becomes even more apparent when considering the time-dependent discounted first SRS as a cumulative second SRS. Tailoring SRS retreatment protocols based on NTCP modeling can potentially enhance therapeutic efficacy.