Purpose/Objective(s): Patients (pts) receiving head and neck cancer radiotherapy often experience toxicity that impacts quality of life. Compared to intensity-modulated radiotherapy (IMRT), proton beam therapy (PBT) can improve normal tissue sparing; however, the clinical impact of PBT has been difficult to quantify in the absence of randomized data. A model-based approach may be an alternative method to quantify potential clinical gains, helping guide appropriate patient selection. The aim of this study was to generate normal tissue complication probability (NTCP) models in pts treated with either PBT or IMRT for oropharynx cancer (OC), and to determine results (PBT vs IMRT). Materials/Methods: The study was limited to pts with advanced-stage oropharynx cancer treated with curative intent (PBT, n Z 30; IMRT, n Z 175). Multivariable NTCP models were developed using multivariable logistic regression analysis with backward selection. In the PBT treated pts, an alternative IMRT plan was also made, to serve as a reference to determine the benefit of PBT in terms of NTCP. The models were then applied to the PBT treated pts to compare predicted and observed clinical outcomes (calibration-in-the large). Five binary endpoints were analyzed at 6 months post-treatment: dysphagia grade 2, dysphagia grade 3, xerostomia grade 2, salivary duct inflammation grade 2, and feeding tube dependence. Corresponding toxicity grading was based on CTCAEv4. Paired t-tests and Wilcoxon rank tests were used to compare mean NTCP results for endpoints between PBT and IMRT. Results: The NTCP models developed based on outcomes from all pts were applied to those receiving PBT. For pts receiving PBT, no significant differences were observed between the expected and observed prevalences (Table 1). In addition, the NTCP-values were calculated for the alternative IMRT plans for all PBT treated pts, revealing significantly higher NTCPvalues for the IMRT plans (Table). Proton therapy was associated with statistically significant reductions in the mean NTCP values for each endpoint at 6 months post treatment, with the largest absolute differences in rates of > grade 2 dysphagia and xerostomia (Table 1). Conclusion: This model-based approach demonstrates significant improvements in early treatment-related toxicity with proton therapy compared to IMRT for oropharyngeal cancer. This study demonstrates the value of NTCP model based approaches in comparing predicted patient outcomes when randomized data are not available. Further studies and longer-term follow-up are warranted to confirm whether these benefits associated with proton therapy persist over time and in other groups of head and neck cancer pts.
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