The Study On Dose Calibration And Verification Of CyberKnife Based On Monte Carlo Algorithm

The small fields are used for stereotactic radiosurgery (SRS) for CyberKnife system. There are electrons nonequilibrium caused by small fields and the effects of material and volume of detectors, and it makes the measurement, calculation and evaluation of dosimetry difficult. This study will use Monte Carlo to simulate the Cyberknife system, and intend to establish the third-party model comparing to the algorithms of TPS. It is possible to investigate the dose calibration under non-stardard reference condition and dose verification of specific patient plans.By simulating the physical structure of CyberKnife treatment head using the BEAMnrc and DOSXYZnrc softwares, the comparison between the caculated and measured datas such as PDDs, OARs and output factors (OFs), etc. will be done to determine the beam parameters. And the ideal beam model will be developed to achieve the dose calibration in the non-standard reference field. The dose distribution caculated in the Ray-tracing, Monte Carlo algorithm of TPS and developed Monte Carlo simulation method for 30 patient plans, which included 10 head, lung and liver cases in each,were analyzed. The y analysis with the combined 3mm/3% criteria would be introduced to quantitatively evaluate the difference of the accuracy between three algorithms, and the dose verification method based on Monte Carlo simulation (the third-party tool) would be explored.More than 90% of the global error points were less than 2% for the comparison of the calculated and measured datas such as PDD and OAR curves after determining the mean energy of beam and the full width at half maximum of its intensity. The relative ideal Monte Carlo beam model had been established for 12 collimators from 5mm to 60mm. It was achieved the dose calibration under the non-stardard field. Based on the quantitative evaluation of dose accuracy for three algorithms, the results of y analysis shows that the passing rates (84.882±9.666% for head,98.830±1.049% for liver, 98.263±1.867% for lung) in 30 patient plans between Monte Carlo simulation and TPS Monte Carlo algorithms were good. And the passing rates (95.933±3.120%, 99.836±0.334% in each) in head and liver patient plans between Monte Carlo simulation and TPS Ray-tracing algorithms were also good. But the difference of DVHs in lung plans between Monte Carlo simulation and TPS Ray-tracing algorithms was obvious, and the passing rate of y criteria (51.263±38.964%) was low. It is feasible that Monte Carlo simulation was used for verifying the dose distribution of patient plans in CyberKnife.Monte Carlo simulation algorithm developed in the CyberKnife system of this study can be used as a reference tood for the third-party software, which plays an important role in the dose calibration of non-standard field and dose verification of patient plans.