Exhibit Hall | Forum 3
Purpose: To develop an independent dose calculation method for Elekta Unity MR-Linac and test its accuracy and uncertainty in water phantom and patient cases.
Methods: An independent dose calculation method based on Monte Carlo simulation was developed using the TOPAS/Geant4 Toolkit to simulate the transport of photons or electrons in strong magnetic field. A full model of the head including target, primary collimator, MLC, diaphragm, and any transmission layers in the beam path such as cryostat annulus, helium, couch, and coils were simulated. The parameters of electron beam source were tuned to adjust the calculated dose profiles and PDD curves for best accordance with measurement. The field of 10 x 10 cm² with 100 MU measured at 133.5 cm SSD during the MR-Linac commissioning was used for the parameter tunning. A phase space file above MLC was then generated to serve as a new source that is independent of variations in subsequent beam limiting devices per segment. The accuracy of dose calculation was validated by comparing the dose profiles and PPD curves between calculated and measured dose for field sizes ranging from 5 x 5 cm² to 57 x 22 cm² in water. Statistical uncertainty was calculated with 2.24 x 10⁸ histories for each standard field. Three simulations for prostate, live and pancreas was conducted to evaluate the method.
Results: The errors of the independent dose calculation for all field sizes in water can be less than 1% with uncertainty < 0.8% with sufficient histories. Gamma analysis 2%/2mm criteria were satisfied with 90%-95% pass rate between calculated and planned patient data.
Conclusion: With sufficient accuracy and precision, this method is a powerful tool for dosimetric analysis of adaptive treatment using Elekta Unity MR-Linac.