Stopping-power ratio of mouthpiece materials for charged-particle therapy in head and neck cancer

Background: In charged-particle therapy treatment planning, CT numbers of a patient’s body are converted into stopping power ratios (SPRs) using a CT-number-to-SPR conversion table constructed for standard human tissues. Since artificial devices used in treatments of head and neck cancer, such as mouthpieces, do not fit into the CT-number-to-SPR conversion table, they may deteriorate treatment accuracy. In this study, the SPRs of five mouthpiece materials were measured, and the error in predicted SPRs based on the conversion table was investigated. Methods: SPRs of five mouthpiece materials were predicted from their CT numbers using a calibrated conversion table. Independently, the SPRs of the materials were measured by the Bragg-peak shift of a carbon-ion beam passing through the materials. The errors in SPR of the materials were determined as the difference between the predicted and measured SPRs. Results: The SPRs (error) of the ethylene-vinyl acetate copolymers (EVAs), Nipoflex 710 TM and Bioplast TM , were 0.997 (0.023) and 0.982 (0.007), respectively. The SPRs (error) of the vinyl silicon impression material (Exafine putty type TM ), the light curable resin (Clear Photoreactive Resin for Formlabs 3D printers TM ), and the bis-acrylic resin (Tempsmart TM ) were 1.517 (0.134), 1.161 (0.068), and 1.26 (0.101), respectively. Conclusions: The EVA Bioplast TM had the minimum SPR error among the five tested materials, indicating the highest human-tissue equivalency. If other artificial materials such as Exafine putty type TM are used as mouthpieces, it is recommended that their SPRs be overwritten by the correct values in treatment planning processes.