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Developing a Source Model for the Elekta Unity MR-Linac Using a Grid-Based Boltzmann Solver

S Nowak1*, D Hyer1 , R Yang2 , J St-Aubin1 , (1) University of Iowa, Iowa City, IA, (2) Cross Cancer Institute, Edmonton, AB


(Sunday, 7/14/2019) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 7

Purpose: To develop a radiation source model for a grid-based Boltzmann solver (GBBS) that can be applied to the 7.2 MV flattening filter free (FFF) Elekta Unity MR-Linac.

Methods: A GBBS, which includes the effects of strong magnetic fields, was implemented in MATLAB to calculate dose deposited in a 1.5 T perpendicular magnetic field. The energy spectrum of our source was based on a 6 MV Linac that was scaled to 7.2 MV and discretized into 24 energy bins. The relative weight of each energy bin was optimized by minimizing the difference between the calculated and measured percent depth dose (PDD) for field sizes ranging from 5x5 cm² to 57x22 cm². The fluence profile was modeled using (1) a weighted sum of two Gaussians to represent the in-field fluence, (2) a Gaussian kernel convolved with the primary fluence to model the geometric penumbra, and (3) an offset value to represent leakage through the collimator jaws. The full-width-half-maximum (FWHM) of each Gaussian, along with their relative amplitudes, was optimized by minimizing the percent difference between the calculated and measured profiles over the same field sizes as the PDDs.

Results: The PDDs matched to within 0.6% of the maximum value beyond the depth of maximum dose for all field sizes. The off-axis (OAX) profiles matched to within 3% of the central-axis (CAX) value for the central beam region. Additionally, the OAX profiles in the penumbra region were reproduced to within 3 mm of the measured values for all field sizes.

Conclusion: The results indicate that our current mathematical model can be used to accurately represent the radiation source for the Elekta Unity MR-Linac. Future work will focus on refining the penumbra model and developing a model for the tongue-and-groove and the rounded leaf ends of the multi-leaf collimator (MLC).


Radiation Therapy, Dose, Modeling


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined dose computation

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