Background: ADT with salvage RT improves survival for men with PSA recurrence after RP. For high risk PSA recurrence, current standard ADT duration is up to 2 years with RT; shortening but intensifying systemic therapy may improve outcomes. The STREAM trial showed 6 mo enzalutamide with ADT/RT had 3-year progression free survival (PFS) of 53% in a high risk population, including lymph node (LN) positive. Given docetaxel (doce) improves survival in mHSPC, we evaluated the combination of salvage RT, ADT/apalutamide (apa) and doce in this setting. Methods: STARTAR is a multicenter investigator initiated phase 2 trial for salvage treatment of PSA recurrent PC post-RP, conducted in the DOD Prostate Cancer Clinical Trials Consortium. Key inclusion criteria included Gleason 7 with T3/positive margin/N1 or Gleason 8-10 PC, PSA relapse <4 years post-RP (inclusion PSA 0.2-4 ng/mL), and <4 positive LNs, standard imaging negative. Patients (pts) received ADT with apa for 9 months, RT (66-74 Gy to the prostate bed +/- pelvic LNs over 6-8 wks) starting wk 8, and then completed 6 cycles of concurrent doce 75mg/m2 q3 wks. The primary endpoint was 36 month PFS, defined as composite of freedom from PSA>0.2 + post-RT nadir with subsequent rise, clinical progression, other therapy start, or death, among pts with testosterone (T) recovery (>100ng/dL). Kaplan-Meier estimates for PFS were used to determine landmark 24 mo and 36 mo rates and compared to PFS rates from prior trials using a binomial test. Results: From 3/2018 to 2/2020, 39 pts were enrolled. As of 12/2022 data cutoff, median follow up was 36 months. Baseline characteristics: Gleason 7 in 54%; Gleason 8-10 in 46%; 23% LN positive; median PSA 0.58 ng/mL (range 0.21-3.40), and median time from RP 7.6 mo (range 2-98). 37 pts (95%) and 23 pts (62%) completed at least 1 and all 6 cycles doce, respectively. All pts achieved undetectable PSA nadirs. At 24mo and 36mo, PFS rates were 84% and 72%, respectively, with 95% pts recovering T at 36 mo. Compared to 40% historical PFS and 53% STREAM PFS rates, the 72% 36-mo PFS rate was statistically significantly improved (p<0.001 and p=0.013, respectively). Common any-grade adverse events (AEs) included 98% hot flashes, 88% fatigue, 77% alopecia, 57% dysgeusia, and 53% rash (28% grade 1; 15% grade 2, 10% grade 3), with 5% febrile neutropenia. Conclusions: In this first phase 2 trial of ADT, apa, salvage RT, and 6 cycles of doce for high risk PSA recurrence, the primary endpoint 3-year PFS rate improved to 72%, indicating durable remissions beyond historic controls. Intensifying systemic treatments in the non-metastatic hormone sensitive but high risk salvage setting may be feasible and efficacious, with fewer pts experiencing cancer progression over time. Clinical trial information: NCT03311555.