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Comparative effectiveness of proton therapy versus photon therapy as part of concurrent chemoradiotherapy for locally advanced cancer.

6521 Background: Concurrent chemo-radiotherapy is standard-of-care curative treatment for many cancers, but is associated with substantial morbidity. Proton therapy may increase the tolerability or effectiveness of concurrent chemo-radiotherapy by reducing radiation to normal tissues. Methods: We conducted a comparative effectiveness study of adult non-metastatic cancer patients treated with curative intent with proton chemo-radiotherapy vs. photon chemo-radiotherapy from 2011-2016 at the University of Pennsylvania. Re-irradiation and disease sites treated with photon-only therapy were excluded. Data on adverse events and survival was gathered prospectively. Primary endpoint was 90-day adverse events associated with unplanned hospitalizations (CTCAEv4 grade ≥3 adverse events). Secondary endpoints included decline in ECOG performance status during treatment, 90-day grade ≥2 adverse events, disease-free survival (DFS) and overall survival (OS). Modified Poisson regression models with inverse propensity score weighting were fit for both outcomes. Propensity scores were estimated using an ensemble machine-learning approach. Results: 1,483 patients were included (391 proton/1,092 photon). Proton patients were significantly older (median 66 vs. 61), had less favorable Charlson-Deyo comorbidity scores (median 3.0 vs. 2.0), and had lower integral radiation dose to tissues outside the target (p < 0.05 for all). Baseline toxicity and performance status were similar (p > 0.05). In propensity score weighted-analyses, proton chemo-radiotherapy was associated with significantly lower relative risk (RR) of 90-day grade ≥3 adverse events [11.5%(95%CI 8.3-14.7%) for protons; 27.6%(95%CI 24.9-30.2%) for photons; RR 0.31, 95%CI 0.15-0.66, p < 0.01]; 90-day grade ≥2 adverse events (RR 0.78, 95%CI 0.65-0.93, p < 0.01); and decline in performance status during treatment (RR 0.51, 95%CI 0.37-0.71, p < 0.01). There was no difference in DFS or OS. Conclusions: In adults with locally advanced cancer, proton chemo-radiotherapy was associated with significantly reduced acute adverse events causing unplanned hospitalizations with similar disease-free and overall survival.