Displacement of the Tongue Base and Soft Palate Because of Breathing Patterns During Radiation Therapy for Head and Neck Cancer

Intra-fractional reproducibility is an important factor in radiation therapy (RT). More accurate reproducibility is required, especially in the case of high-precision RT, such as particle therapy and intensity-modulated RT. To maintain reproducibility and minimize positional errors, patients should be irradiated in the same manner as at the time of treatment-planning computed tomography (CT). Patient motion errors are managed using an immobilization device. For head and neck tumors, an intraoral device is used to fix the position of the tongue and control its motion.1Ikawa H Koto M Ebner DK et al.A custom-made mouthpiece incorporating tongue depressors and elevators to reduce radiation-induced tongue mucositis during carbon-ion radiation therapy for head and neck cancer.Pract Radiat Oncol. 2018; 8: e27-e31https://doi.org/10.1016/j.prro.2017.10.009Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar Naturally, patients breathe during RT. There are three breathing patterns: nasal, oral, and oronasal. However, no breathing pattern is generally specified during RT. The position of the organs in the oral cavity is anatomically and physiologically altered by the way patients breathe. Examples of treatment-planning CT images of the same patient with different breathing patterns are shown in Figure 1. During nasal breathing, the soft palate is lowered caudally and the base of the tongue is raised and attached, creating an airway in the nasal cavity (Fig. 1a). In contrast, during oral breathing, the soft palate is elevated and the base of the tongue is lowered caudally and separated, creating an airway to the mouth (Fig. 1b). Therefore, the positions of the base of the tongue and soft palate differed significantly depending on breathing patterns. In head and neck cancer cases, if the breathing patterns differ between treatment-planning CT and irradiation, reproducibility may decrease and intra-fractional error may increase. In this example case, a maximum craniocaudal positioning error of 10 mm for the base of the tongue and 7 mm for the soft palate was observed between nasal and oral breathing. Therefore, in head and neck cancers where the target involves the base of the tongue or soft palate, different breathing patterns may result in irradiation failure according to the treatment plan, resulting in local recurrence and unexpected side effects. The ICRU report 62 described the internal margin associated with organ movement due to respiration.2International Commission on Radiation Units and Measurements ICRU Report 62.in: Prescribing, Recording, and Reporting Photon Beam Therapy (Supplement to ICRU Report 50), Bethesda, MD ICRU, 1999Google Scholar However, breathing patterns during RT for head and neck tumors have not been described. Further, the effects of different breathing methods on RT remain unknown. Therefore, it is necessary to identify errors in dose distribution due to different breathing patterns during RT and to make recommendations for the unification of breathing patterns. Hiroaki Ikawa: Writing – original draft, Methodology, Data curation, Conceptualization. Masashi Koto: Writing – review & editing, Supervision. The authors declare that they have no known competing financial interests or personal relationships that may influence the work reported in this study. None