Investigation of Reduction of the Uncertainty of Monte Carlo Dose Calculations in Oncor® Clinical Linear Accelerator Simulation Using the DBS Variance Reduction Technique in Monte Carlo Code BEAMnrc

To study the benefits of Directional Bremsstrahlung Splitting (DBS) dose variance reduction technique in BEAMnrc Monte Carlo (MC) code for Oncor® linac at 6MV and 18MV energies.
A MC model of Oncor® linac was built using BEAMnrc MC Code and verified by the measured data for 6MV and 18MV energies of various field sizes. Then Oncor® machine was modeled running DBS technique, and the efficiency of total fluence and spatial fluence for electron and photon, the efficiency of dose variance reduction of MC calculations for PDD on the central beam axis and lateral dose profile across the nominal field was measured and compared.
With applying DBS technique, the total fluence of electron and photon increased in turn 626.8 (6MV) and 983.4 (6MV), and 285.6 (18MV) and 737.8 (18MV), the spatial fluence of electron and photon improved in turn 308.6±1.35% (6MV) and 480.38±0.43% (6MV), and 153±0.9% (18MV) and 462.6±0.27% (18MV). Moreover, by running DBS technique, the efficiency of dose variance reduction for PDD MC dose calculations before maximum dose point and after dose maximum point enhanced 187.8±0.68% (6MV) and 184.6±0.65% (6MV), 156±0.43% (18MV) and 153±0.37% (18MV), respectively, and the efficiency of MC calculations for lateral dose profile remarkably on the central beam axis and across the treatment field raised in turn 197±0.66% (6MV) and 214.6±0.73% (6MV), 175±0.36% (18MV) and 181.4±0.45% (18MV).
Applying dose variance reduction technique of DBS for modeling Oncor® linac with using BEAMnrc MC Code surprisingly improved the fluence of electron and photon, and it therefore enhanced the efficiency of dose variance reduction for MC calculations. As a result, running DBS in different kinds of MC simulation Codes might be beneficent in reducing the uncertainty of MC calculations.