Proton Beam Therapy for Locally Advanced Head and Neck Tumors: An Analysis of Dosimetric and Clinical Outcomes

Objective: Locally advanced tumors of the head and neck region often lie in close proximity to critical organs at risk (OARs). Providing effective treatment coverage to these malignancies while minimizing radiation dose to surrounding OARs is advantageous. Our aim is to compare dosimetric data of OARs from proton beam therapy (PBT) plans to volumetric modulated arc therapy (VMAT) treatment plans, and to evaluate clinical outcomes in patients treated with PBT. Methods: We identified patients with locally advanced head and neck tumors treated with PBT at our institution from 2016 to 2019. Study endpoints included mean and maximum doses for the OAR structures for each treatment plan, overall survival, time to local-regional or distant progression, and presence of acute and late toxicities. Mean and maximum doses to OAR structures were compared between treatment modalities using a paired Wilcoxon signed-rank test. P-values <0.05 were considered significant. Results: A total of 42 patients were identified. Clinical target volume coverage was > 95% for both PBT and VMAT plans. PBT plans showed a significant reduction to the mean doses to all OARs, and max doses to most OARs (P < 0.05). The largest reduction mean dose was seen in the contralateral cochlea and parotid glands at 71% and 75%, respectively. Median follow-up was 27 months. Overall survival at 4 years was 44.75%. Freedom from local-regional progression was 73.28% at 2 years. The majority of patients developed Common Terminology Criteria for Adverse Events (CTCAE) grade I dermatitis, mucositis, or both. Conclusions: PBT resulted in meaningful dose reductions to OARs while maintaining comparable target coverage when compared with VMAT plans. Further refinements to proton therapy may have the potential to further minimize dose to critical structures.