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.

Diffusion tensor imaging for evaluation of white matter tracts is used with magnetic resonance spectroscopy (MRS) to improve management of diffuse intrinsic pontine glioma (DIPG). Changes in the apparent diffusion coefficient (ADC), fractional anisotropy (FA), and tumor metabolite ratios have been reported after initial radiation for DIPG, but these markers have not been studied sequentially in patients undergoing reirradiation for progressive DIPG. Here, we report a case series of 4 patients who received reirradiation for progressive DIPG on a prospective clinical trial in which we evaluated quantitative changes in FA, ADC, and tumor metabolites and qualitative changes in white matter tracts. The median reirradiation dose was 25.2 Gy (24-30.8 Gy). Fiber tracking was performed using standard tractography analysis. The FA and ADC values for the corticospinal and medial lemniscus tracts were calculated before and after reirradiation. Multivoxel MRS was performed. Findings were correlated with clinical features and conventional MRI of tumors. All patients had an initial response to reirradiation as shown by a decrease in tumor size. In general, FA increased with disease response and decreased with progression, whereas ADC decreased with disease response and increased with progression. At second progression, the FA fold change relative to values during disease response decreased in both patients with available imaging at second progression. Visualization of tracts demonstrated robust reconstitution of previously disrupted paths during tumor response; conversely, there was increased fiber tract disruption and infiltration during tumor progression. The MRS analysis revealed a decrease in choline:creatinine and choline:N-acetylaspartate ratios during tumor response and increase during progression. Distinct changes in white matter tracts and tumor metabolism were observed in patients with DIPG undergoing reirradiation on a prospective clinical trial. Changes related to tumor response and progression were observed after 24 to 30.8 Gy reirradiation.