(P024) Brainstem and Oral Cavity Sparing in the Adjuvant Treatment of Parotid Cancer: A Comparative Analysis of Volumetric Modulated Arc Therapy (VMAT) and Multifield Optimization Intensity-Modulated Proton Therapy (MFO-IMPT)

Natalie Geier, MD, Anthony Mascia, PhD, April Ward, CMD, Bradley Huth, MD, Michelle Mierzwa, MD, Kevin Redmond, MD, William Barrett, MD, Jonathan Mark, MD, Vinita Takiar, PhD, MD; University of Cincinnati Medical Center; University of Michigan

BACKGROUND: Treatment of parotid gland tumors with radiotherapy (RT) can result in xerostomia and dysgeusia. Minimizing oral cavity (OC) and brainstem dose results in improved appetite and less fatigue, respectively. We performed a dosimetric analysis evaluating target coverage and sparing of organs at risk (OARs) with volumetric modulated arc therapy (VMAT) and multifield optimization intensity-modulated proton therapy (MFO-IMPT) in patients receiving postoperative RT to the parotid bed.

METHODS: The study population included 11 patients, 7 of whom received adjuvant RT and 4 of whom who received adjuvant chemoradiotherapy to the parotid gland and ipsilateral neck following surgical resection between 2012 and 2014 at a single institution. VMAT and MFO-IMPT plans were reconstructed for all patients with OARs delineated per Radiation Therapy Oncology Group trial 0920. All plans underwent institutional quality assurance peer review. Quality-of-life data (QOL European Organisation for Research and Treatment of Cancer HN35) were also obtained. 

RESULTS: All plans achieved institutional planning objectives and were clinically acceptable. The mean absolute clinical target volumes getting 10% and 30% of the prescription dose were 1,568 cc and 1,013 cc, respectively, for MFO-IMPT plans and 4,354 cc and 1,228 cc, respectively, for VMAT plans (P < .05 for all). MFO-IMPT planning reduced spinal cord (SC), brainstem, contralateral parotid, and OC dose compared with VMAT. Average SC maximum dose (D_max) was 60% lower for MFO-IMPT (13.7 Gy vs 33.0 Gy; P < .05). Average brainstem D_max was halved with MFO-IMPT (11.7 Gy vs 24.1 Gy; P < .05); mean contralateral parotid dose was only 0.05 Gy for MFO-IMPT (7.9 Gy for VMAT; P < .05). Mean OC dose was reduced by 80% with MFO-IMPT (4.0 Gy vs 19.2 Gy; P < .05). Of six patients with QOL data available, five noted fatigue and four experienced decreased appetite.

CONCLUSION: MFO-IMPT planning for parotid cancers reduces low-dose radiation volumes and significantly reduces SC, brainstem, contralateral parotid, and OC dose. Reducing brainstem dose may result in less fatigue, while decreasing OC and uninvolved parotid gland dose may improve appetite. These parameters should be examined prospectively.

Proceedings of the 98th Annual Meeting of the American Radium Society - americanradiumsociety.org