Study of immobilization accuracy with different mouthpiece shapes in radiation therapy for head and neck cancer

Introduction: During radiation therapy for head and neck cancers, oral devices such as mouthpieces or stents may be used to reduce positioning errors. However, no reports to date have discussed the immobilization accuracy associated with different mouthpiece shapes. In this study, we evaluated t he immobilization accuracy of two different mouthpiece shapes. Materials and Methods: Seventeen patients who underwent radiation therapy for head and neck cancers at the Department of Radiology of Nikko Memorial Hospital from October 2017 to June 2023 were included. Systematic error, random error, and set-up margin, determined when the thermoplastic mask and headrest were fitted and prior to the start of radiation therapy, were compared between patients in whom a full mouthpiece was used and patients in whom an anterior mouthpiece was used. Anterior -posterior, superior-inferior, left‒right, yaw, roll, and pitch measurements were performed. The set-up margin was calculated from the systematic error and random error using the van Herk margin formula. Statistical analysis was performed using the Mann-Whitney U test. Results: With the exception of yaw and pitch prior to the start of treatment, random errors tended to be smaller in the full mouthpiece group than in the anterior mouthpiece group. Ca lculation of the set-up margin showed that when the thermoplastic mask and headrest were fitted, the superior‒infer ior measurement was smaller in the full mouthpiece group at 0.91 mm. The anterior‒posterior measurement prior to t he start of treatment was also smaller in the full mouthpiece group at 0.61 mm. The Mann-Whitney U test showed a significant difference between the left‒right and yaw measurements prior to the start of treatment. Conclusion: Immobilization accuracy was compared between two di fferently shaped mouthpieces. The full mouthpiece was considered more useful than the anterior mouthpiece because of its ability to reduce error.