Adaptation of Stochastic Microdosimetric Kinetic Model for Charged-Particle Therapy Treatment Planning

Purpose: The microdosimetric kinetic (MK) model underestimates the cell-survival fractions for high linear energy transfer (LET) and high dose irradiations. To address the issue, the MK model was extended to the stochastic microdosimetric kinetic (SMK) model. The purpose of this study is to modify the SMK model to make it applicable to scanned charged-particle therapy treatment planning. \nMethods: To estimate the radiation induced cell-survival fractions based on the SMK model, the stochastic natures of the cell-nucleus specific energy z_n as well as the (subnuclear structure) domain specific energy z_d must be considered, which made the SMK-based computations time consuming and memory intensive. By introducing the single-event dose-averaged cell-nucleus specific energy in the SMK formalism, the stochastic nature of z_n was reflected onto the estimated cell-survival fractions. The accuracy of the modified SMK model was examined through the comparison between the estimated and the measured survival fractions of human salivary gland tumor (HSG) cells and V79 cells. We then implemented the modified SMK model into the in-house treatment planning software for scanned charged-particle therapy to validate its applicability in clinical practice. As examples, treatment plans of carbon-ion beams were made for an orbital tumor case. \nResults: The modified SMK model could reproduce the measured cell-survival fractions more accurately comparing to the MK model especially for high-LET and high-dose irradiations. The in silico study on an orbital tumor case validated the modified-SMK-model-based computations for scanned charged-particle therapy treatment planning. Neither the inverse planning nor the forward biological-dose calculations led to a remarkable prolongation in computation time by the modified SMK model as compared to the MK model. \nConclusion: The modified SMK model offers the accuracy and simplicity required in treatment planning of scanned charged-particle therapy for wide LET and dose ranges.AAPM 60th Annual Meeting & Exhibitio