Author: Matthew J Ferris, Jason K Molitoris 👨🔬
Affiliation: Department of Radiation Oncology, University of Maryland School of Medicine, University of Maryland 🌍
Purpose: Anatomy changes often happen in head-and-neck (HN) patients treated with proton radiation therapy. These changes can result in deteriorations of target dose coverage and increased dose to organs at risk (OARs). We developed a method to quantify anatomy changes using daily setup cone beam CT (CBCT). This strategy assists with prediction of need for adaptive replan.
Methods: Twenty-two HN replan cases treated at our proton center in 2024 were selected for this study. Replans were done at least partially related to decreased weight or tumor volumes. Rigid registration parameters were used to transfer the target volume from the planning CT to each CBCT. The proton range (water equivalent thickness) was calculated between the proton source and each voxel on the target volume surface . When the voxels were out of the body and/or in the air cavity due to weight loss and tumor shrinkage, we recorded the number of such voxels and the change of proton ranges compared to those in the 1st day CBCT .
Results: The range change at those voxels was from 3.5% and 12.6%, with mean 7.0% and stdev 2.6% for the 22 replan cases. The 3.5% matches our robust optimization on CT density curve uncertainty (3.5%), but 21of the adaptations were triggered when the range changes were larger than 3.5%. This was because the concerned OARs were not very close to the target, and thus the weight loss and tumor shrinkage did not immediately increase the OAR dose to a concerned level.
Conclusion: Our method can quantitatively determine shrinkage in patient circumference or tumor volume for HN patients. By using the daily range changes, the number of weekly QACTs can be potentially reduced and proton adaptation can be predicted.