A Study on Building a Monte Carlo Model of G4 CyberKnife Based on the Third-party Softwares

摘要

Purpose: To study the beam characteristics of CyberKnife system, especially the features in standardization and calibration of small-field dosimetry under non-standard condition, this research utilized Monte Carlo method to simulate the transporting process of beams in G4 CyberKnifeand compared the results with the outcomes of actual measurement so that the accuracy of the established model can be verified. Materials and Methods: BEAMnrc/DOSXYZnrc Monte Carlo system was used for simulating the physical structure of G4 CyberKnife model (containing 12 collimatorsof different sizes). With the adjustment of parameters (the average energy, full width at half maximum of intensity distribution) of electron beam, data of dose distribution for the fields (percentage depth dose, off-axis ratio, output factor, beam quality, correction factor, etc.) in thewater phantom could be acquired and compared with the actual measurement so as to determine the parameters of electron beam in the simulated model. Results: In the comparisons of percentage depth dose, the errors with more than 90% of the points were smaller than 1% (except 5mm and 7.5mmcollimators). And for the off-axis ratios, the errors with more than 90% of the points were smaller than 1% (except 5mm, 7.5mm and 10mm collimators). The values of beam output factors in a Monte Carlo system were consistent with the measured values when the collimator sizes were larger than20mm, and when the collimator size were smaller than 15mm, a certain deviation for output factor existed. The value of beam quality in Monte Carlo model was 0.639, which was close to that of the measured beam quality, 0.635. Conclusion: The parameters of electron beam were finally determined(the average energy was 6.9MeV and the full width at half maximum of intensity distribution was 0.4 cm). The Monte Carlo model established in this research was able to accurately simulate the beam transporting process of CyberKnife and thus might lay the foundation for the further research.