Author: Kristy K. Brock, Austin Castelo, Yulun He, Zhongxing Liao, Rebecca Lim, Radhe Mohan, Caleb O'Connor, Tien T Tang, Uwe Titt 👨🔬
Affiliation: University of Washington, The University of Texas MD Anderson Cancer Center, Department of Imaging Physics, The University of Texas MD Anderson Cancer Center 🌍
Purpose: Conformal dose distributions in proton radiotherapy promise to reduce normal tissue toxicity such as radiation-induced pneumonitis, but this has not been fully realized in clinical trials. To further investigate dose and toxicity, we employ voxel-based normal tissue evaluation techniques such as ventilation maps throughout treatment. We hypothesize the following: ventilation change after 1 week of treatment (WK1) predicts for ventilation change at the end of treatment (EOT), and ventilation changes at EOT are different between protons and photons.
Methods: For 46 photon and 23 proton lung cancer patients, 4DCT-based ventilation maps were generated using stress-based methods at planning, WK1, and EOT. Voxel-wise ventilation change from planning to WK1 and EOT was calculated and binned by dose, and median ventilation change at WK1 and EOT was calculated across all patients in each dose bin. Patients were stratified into 4 groups based on modality and increase vs. decrease in ventilation in a majority of dose bins at WK1. Mann-Whitney U tests were performed to determine if median ventilation change at each dose bin was significantly different from zero.
Results: 22 photon and 9 proton patients showed decreased ventilation at WK1; of those patients, this was predictive of ventilation decrease at EOT (77% photon and 78% proton patients). The decrease in ventilation was significant at WK1 and EOT. 24 photon and 14 proton patients increased in ventilation at WK1 with 54% and 71% also increased in ventilation at EOT, respectively. In this group, WK1 ventilation increased significantly (p<0.05) for both modalities but only proton patients experienced significant increases at EOT.
Conclusion: WK1 ventilation change was predictive of EOT ventilation change. The significant increase in ventilation at EOT in proton but not photon patients may indicate reduced normal tissue toxicity from protons. Further work is needed to correlate ventilation change with pneumonitis development.