Author: Min Cheol Han, Chae-Seon Hong, Changhwan Kim, Dong Wook Kim, Hojin Kim, Jin Sung Kim, HO Lee, Sac Lee, Yongdo Yun 👨🔬
Affiliation: Department of Radiation Oncology, Yonsei Cancer Center, Heavy Ion Therapy Research Institute, Yonsei University College of Medicine 🌍
Purpose: Carbon ion radiotherapy (CIRT) offers the advantage of a greater relative biological effectiveness (RBE) weighted dose, which must be integrated into the treatment planning system (TPS). This study presents the development of a CIRT TPS at Yonsei Cancer Center (YCC), enabling the optimization of spot weights using pencil beam scanning (PBS) for RBE weighted dose calculation.
Methods: The modified microdosimetric kinetic model (mMKM) was employed to convert the physical dose into RBE weighted dose using z1d* information. TOPAS Monte Carlo simulations fitting to the measured data of CIRT at YCC were used to generate beam kernels for physical and z1d* dose across all energy layers, which constructed dose influence matrices for PBS-based plan optimization. Siddon’s ray tracing algorithm guided beam paths in generating dose influence matrices and accounting for the rotation of gantry angles. The spot weight optimization was performed by the steepest gradient method, accelerated by the ADAM optimizer, which accommodates uniform, minimum/maximum, and dose-volume histogram (DVH) constraints. The spot weights optimized for a water phantom case were re-calculated by TOPAS and a commercial TPS (RayStation) to validate the developed system.
Results: The proposed system fully automated all procedures from generating dose influence matrices to optimizing the PBS-based plans. For the water plan, it achieved the gamma passing rates (GPR) of 98.02% and 94.80% for physical dose, and 93.75% and 94.06% for RBE weighted dose, compared to TOPAS and RayStation (3%/3 mm criteria). These discrepancies may arise from differences in beam kernel generations and beam path guidance using Siddon’s ray tracing, which could be further refined.
Conclusion: This work presented that the new TPS for PBS-based CIRT was successfully developed and assessed, with potential for clinical application following further refinements and validations.