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Simple Summary: Roughly 25% of non-small cell lung cancer (NSCLC) patients subsequently experience isolated locoregional recurrence following definitive radiation therapy (RT). For these scenarios, thoracic reirradiation (re-RT) has become an increasingly common consideration, with improved feasibility due to technological advancements that have made it safer to deliver high doses of RT in the retreatment setting. Proton beam therapy (PBT) is an attractive choice for re-RT due to its ability to spare radiation to adjacent previously treated normal tissues. Here, we evaluate outcomes with PBT for the definitive re-RT of recurrent NSCLC within the previously irradiated thorax. Background/Objectives: This retrospective study evaluates outcomes of 66 patients who underwent reirradiation (re-RT) with proton beam therapy (PBT) for recurrent non-small cell lung cancer. Methods: Toxicity was scored via the CTCAE v5.0, and outcomes estimated using the Kaplan–Meier method, with associations evaluated via Cox proportional hazards and logistic regression analyses. Results: Patients were treated to a median re-RT prescription of 66 Gy/33 fxs (BED10 = 79 Gy; IQR: 71–84 Gy) at an interval of 1.4 years from prior RT. Half (50%) received concurrent chemotherapy. At 14 months follow-up, the median OS and PFS were 5 months (95%CI: 13–17) and 12.5 months (95%CI: 10–15), respectively. On multivariable analysis, a higher RT dose (BED10 > 70 Gy) [HR0.37; 95%CI: 0.20–0.68, p = 0.001] and concurrent chemotherapy (HR0.48; 95%CI: 0.28–0.81, p = 0.007) were associated with improved PFS, while treatment site overlap was adversely associated (HR1.78; 95%CI: 1.05–3.02, p = 0.031). The median PFS for definitive RT with concurrent chemotherapy (n = 28), definitive RT alone (BED10 > 70 Gy) [n = 22], and lower prescription RT (BED10 < 70 Gy) [n = 16] was 15.5 months (95%CI: 7.3–23.7), 14.1 months (95%CI: 10.9–17.3), and 3.3 months (95%CI: 0–12.3), respectively (log-rank, p = 0.006), with corresponding 2-year estimates of 37% (±9), 18% (±8), and 12.5% (±8), respectively. The incidence of Grade 3+ toxicity was 10.5% (6% pulmonary; 3% esophageal; and 1.5% skin), including one Grade 4 bronchopulmonary hemorrhage but no Grade 5 events. Cases with central site overlap had higher composite Dmax to the esophagus (median 87 Gy [IQR:77–90]), great vessels (median 120 Gy [IQR:110–138]), and proximal bronchial tree (median 120 Gy [IQR:110–138]) as compared to other cases (p ≤ 0.001 for all). However, no significant associations were identified with Grade 3+ events. Conclusions: Thoracic re-RT with PBT is an option for recurrent NSCLC with acceptable outcomes and toxicity for select patients. When feasible, higher prescription doses (BED10 > 70 Gy) should be delivered for definitive intent, and concurrent chemotherapy may benefit individual cases. [ABSTRACT FROM AUTHOR]