Author: Hao Gao, Jiayue Han, Wangyao Li, Yuting Lin, Jufri Setianegara, Aoxiang Wang, Yanan Zhu 👨🔬
Affiliation: Department of Biomedical Engineering, Huazhong University of Science and Technology, Department of Radiation Oncology, University of Kansas Medical Center 🌍
Purpose: Proton therapy leverages Bragg-peak-based dose delivery to achieve ultra-high-dose-rate FLASH with patient-specific range modulators (PSRM). Current proton FLASH (pFLASH) planning typically involves optimizing a multi-energy IMPT plan for beam intensities, followed by converting it into a single-energy pFLASH plan using PSRM optimization. However, the inherent interdependence between beam intensities and PSRM remains underexplored. This study introduces and experimentally validates a novel alternating optimization method that integrates PSRM and beam intensities to enhance dose-conformal pFLASH plan quality.
Methods: Compared with the conventional FLASH plan generation method, two key innovations of the proposed method are: (1) An initialization method is designed to provide a warm start for subsequent optimization. Specifically, the beam intensity emerging from the range modulators is directly optimized, and the initialization of PSRM and beam intensity for pFLASH is derived accordingly. (2) Alternating optimization for PSRM and beam intensity is applied in an iterative process, which refines both parameters while ensuring that the solution adheres to delivery constraints, such as the minimum monitor unit (MMU).
Results: The proposed new method outperforms the conventional approach for several clinical cases, in terms of: (1) Improved plan quality; (2) Improved robustness to fewer beams and misalignments between beams and pins. For example, the new method had an improvement of optimization objective value from 5.28×10-3 to 4.23×10-3, maximum target dose from 133.5% to 109.8% and conformity index from 0.804 to 0.838 for a prostate case. Experimental results also showed that the measured integrated depth dose (IDD) curve aligned perfectly with the planned IDD, validating the method's effectiveness.
Conclusion: This study proposes a novel alternating optimization method for FLASH radiotherapy. The new method may extend the applicability of PSRM to more complex clinical scenarios, particularly those involving misalignments between beams and pins. Numerical and experimental results demonstrate the robustness of the new method.