Investigation of the Optimal Mouthpiece Material for Proton Therapy of Head and Neck Cancer

Abstract Purpose: To investigate how the materials of mouthpieces used for proton therapy of head and neck cancer affect the dose distribution in surrounding normal tissue by focusing on the CT values of the materials.Methods: Six dental materials were used to measure CT values: temporary relining resin, tissue conditioner, vinyl polysiloxane, thermoplastic ethylene vinyl acetate copolymer splint, silicone rubber impression material, and a composite impression material. Among these materials, three of the dental materials were investigated further: one material with the CT value closest to water, and the materials with the highest and lowest CT values. Based on these results, we investigated the effect of the CT value of the mouthpiece on the dose distribution in 17 cases in which a mouthpiece was used during proton therapy for head and neck cancers, the treatment plans were recalculated by changing the CT values of the mouthpiece to that of the three identified dental materials. For each cancer case, the irradiation dose to normal tissue was calculated for the treatment plans. The evaluation indices were set to the mandible max dose (GyE), the mandible mean dose (GyE), the volume of the mandible irradiated above 60 GyE (mandible V-60GyE), the parotid affected side mean dose (GyE), the parotid unaffected side mean dose (GyE), and the oral mean dose (GyE). The Wilcoxon’s rank sum test was used to analyze the significance of the differences between treatment plans.Results: The temporary relining resin with the CT value closest to water was 36.9 HU, the vinyl polysiloxane with the highest CT value was 985 HU, and the thermoplastic ethylene vinyl acetate copolymer splint with the lowest CT value was -89.7 HU. The maximum absolute difference among the treatment plans per case was 4.18 GyE for the oral mean dose. The radiation dose for the evaluation indices did not differ significantly among the treatment plans. Conclusion: In the range of CT values from -89.7 HU to 985.0 HU covered in this study, the effect of the CT value of the mouthpiece on the dose distribution may be considered to have no clinical impact.