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.