Factors requiring adaptive re-planning in carbon ion radiotherapy for head and neck cancers

Background The anatomical changes during carbon ion radiotherapy (CIRT) for head and neck cancers frequently require plan adaptations with adaptive re-planning. However, the details of the anatomical changes and their rates are not clear. The purpose of this study was to retrospectively evaluate the factors that require re-planning during CIRT for head and neck cancers. Methods Thirty-seven patients who underwent 146 computed tomography (CT) scans and CIRT for head and neck cancers were evaluated retrospectively. To evaluate the re-planning situation, the frequency of the plan adaptation, reasons for plan adaptations, and causes of dose-distribution distortion were evaluated. Dose distributions were evaluated for two cases in which re-planning was carried out due to changes in tumor size and density on the beam path. Results The main cause for dose-distribution distortion was a change in tumor size for 11 of 13 patients who underwent plan adaptation. Of the tumor-size changes, 64% were due to tumor expansion. For two patients, adaptive re-planning on weekly CT images resulted in a dose distribution equivalent to the original treatment plan in terms of target coverage and sparing of organs at risk. Conclusion In CIRT for head and neck cancers, the main cause of dose-distribution distortion requiring re-planning was an increase in tumor size soon after treatment initiation.