Dosimetric comparison of IMPT versus IMRT in unilateral treatment of head and neck cancer

PurposeThere is currently no consensus on the role of proton therapy in head and neck cancers. We conducted a retrospective dosimetric comparison of delivered photon-based intensity modulated radiation therapy (IMRT) plans with simulated intensity-modulated proton therapy (IMPT) plans. Patients and MethodsIn this single-institution retrospective review, we included patients with primary tumors from all head and neck sites treated with unilateral IMRT, who experienced worsened dysphagia and xerostomia symptoms post-radiation. MD Anderson Dysphagia Inventory (MDADI) and Xerostomia Questionnaire (XQ) scores were prospectively collected. We compared target coverage (V95%) for high-dose and low-dose clinical target volumes (CTVs) and maximum/mean doses for organs-at-risk (OARs) between delivered IMRT plans and simulated IMPT plans. Statistical analysis was performed using Wilcoxon signed-rank tests, with Bonferroni-corrected significance level of 0.003. ResultsA total of 23 patients were included in the study. Both IMRT and IMPT plans provided appropriate target coverage of the high-dose CTV (median V95 99.91% for both) and low-dose CTV (median V95 99.71% and 99.90%, respectively). IMPT plans allowed for significant reduction in maximum dose to critical OARs, including the spinal cord (6.4Gy vs 37.3Gy IMRT, p<0.001) and brainstem (5.6Gy vs 33.0Gy IMRT, p<0.001). Furthermore, mean dose to the oral cavity and contralateral pharyngeal constrictors were significantly reduced in IMPT plans (19.7Gy vs 33.6Gy IMRT oral cavity, p<0.001; 20.4Gy vs 26.2Gy IMRT contralateral pharyngeal constrictor, p<0.001). IMPT spared dose to the contralateral parotid (0.04Gy vs 7.6Gy IMRT, p<0.001) and contralateral submandibular gland (1.4Gy vs 15.4Gy, p<0.001). ConclusionIMPT spares dose to OARs compared to IMRT plans in head and neck cancers treated with unilateral radiation. We hypothesize that IMPT can reduce acute and long-term toxicity for these patients, even in locally advanced cancers. Future prospective comparison between these treatment modalities is indicated.