The Impact of Anatomic Change on Pencil Beam Scanning in the Treatment of Oropharynx Cancer

Abstract

Purpose To investigate the potential impact of anatomic change in the treatment of locally advanced oropharyngeal cancer with proton pencil beam scanning. Materials and Methods Ten patients with locally advanced oropharyngeal cancer who previously received intensity-modulated radiation therapy (IMRT) and synchronous chemotherapy underwent replanning by using RapidArc IMRT and proton pencil beam scanning. Deformable image registration deformed the planning computed tomography (CT), target volumes, and organs at risk (OARs) contours onto each weekly cone-beam CT scan. Target and OARs volumes were reviewed and modified. Treatment plans were forward calculated onto each corrected cone-beam CT scan and dose-volume histograms produced for targets and OARs volumes. Results Proton pencil beam scanning compared with RapidArc IMRT achieved lower mean doses to the contralateral parotid gland (14.8 Gy versus 20.6 Gy, P < .05) and oral cavity (31.5 Gy versus 43.0 Gy, P < .001). For proton pencil beam scanning, mean delivered doses to several OARs significantly increased from week 3 or 4 of treatment; this was not observed in RapidArc plans. The respective overall increases in mean delivered doses (average weeks 1 to 7, compared with week 1) to the pharyngeal constrictor muscles, larynx, and oral cavity were 1.0 Gy (SD, ± 1.3), 3.3 Gy (SD, ± 3.3), and 1.7 Gy (SD, ± 1.9). Conclusions Although proton therapy provided dosimetric advantages on initial planning, it was more sensitive to interfraction anatomic changes, resulting in clinically significant increases in delivered doses to several OARs. We would therefore advocate for clinical implementation of methods to measure, account for, and adapt to these changes during the course of treatment in order to fully realize the potential benefits of proton therapy in this setting.