Effect of Mouthpiece Materials for Proton Therapy of Head and Neck Cancer With an Investigation of Material Suitability

ABSTRACT Aim To investigate the effect and suitability of mouthpiece materials for proton therapy (PT) of head and neck cancer (HNC). Methods Computed tomography (CT) numbers and the stopping power ratios (SPRs) of six dental materials were measured. The SPRs were converted to CT numbers, and the materials with the highest, lowest, and closest to the CT numbers of surrounding normal tissue were identified. To investigate how mouthpiece CT numbers affect the dose distribution for HNC, treatment plans of 17 patients were recalculated by changing the mouthpiece CT numbers to these three CT numbers, and doses to normal tissues were measured. Results Vinyl polysiloxane had the highest CT number, and thermoplastic ethylene vinyl acetate copolymer splints had the lowest CT number. Temporary relining resin had the CT number closest to normal tissue. Temporary relining resin had the smallest difference in the CT of the CT image, and the CT converted from SPR. The plans of temporary relining resin did not differ significantly from the actual plan, and the patient with the largest dose difference had a 1.75 gray equivalent (GyE) in the mandible max dose. However, that of vinyl polysiloxane and that of thermoplastic ethylene vinyl acetate copolymer splints differed significantly from the actual plans in several of the measured items, with the maximum difference of 5.43 GyE in the oral mean dose. Conclusion We confirmed that changing the material of the mouthpiece could cause the dose to normal tissues to vary by approximately 5 GyE.