Evaluation of a commercial MRI Linac based Monte Carlo dose calculation algorithm with <scp>geant</scp> 4

<jats:sec><jats:title>Purpose:</jats:title><jats:p>This paper provides a comparison between a fast, commercial, in‐patient Monte Carlo dose calculation algorithm (GPUMCD) and <jats:sc>geant4</jats:sc>. It also evaluates the dosimetric impact of the application of an external 1.5 T magnetic field.</jats:p></jats:sec><jats:sec><jats:title>Methods:</jats:title><jats:p>A stand‐alone version of the Elekta™ GPUMCD algorithm, to be used within the Monaco treatment planning system to model dose for the Elekta™ magnetic resonance imaging (MRI) Linac, was compared against <jats:sc>geant4</jats:sc> (v10.1). This was done in the presence or absence of a 1.5 T static magnetic field directed orthogonally to the radiation beam axis. Phantoms with material compositions of water, ICRU lung, ICRU compact‐bone, and titanium were used for this purpose. Beams with 2 MeV monoenergetic photons as well as a 7 MV histogrammed spectrum representing the MRI Linac spectrum were emitted from a point source using a nominal source‐to‐surface distance of 142.5 cm. Field sizes ranged from 1.5 × 1.5 to 10 × 10 cm<jats:sup>2</jats:sup>. Dose scoring was performed using a 3D grid comprising 1 mm<jats:sup>3</jats:sup> voxels. The production thresholds were equivalent for both codes. Results were analyzed based upon a voxel by voxel dose difference between the two codes and also using a volumetric gamma analysis.</jats:p></jats:sec><jats:sec><jats:title>Results:</jats:title><jats:p>Comparisons were drawn from central axis depth doses, cross beam profiles, and isodose contours. Both in the presence and absence of a 1.5 T static magnetic field the relative differences in doses scored along the beam central axis were less than 1% for the homogeneous water phantom and all results matched within a maximum of ±2% for heterogeneous phantoms. Volumetric gamma analysis indicated that more than 99% of the examined volume passed gamma criteria of 2%—2 mm (dose difference and distance to agreement, respectively). These criteria were chosen because the minimum primary statistical uncertainty in dose scoring voxels was 0.5%. The presence of the magnetic field affects the dose at the interface depending upon the density of the material on either sides of the interface. This effect varies with the field size. For example, at the water‐lung interface a 33.94% increase in dose was observed (relative to the <jats:italic>D</jats:italic><jats:sub>max</jats:sub>), by both GPUMCD and <jats:sc>geant4</jats:sc> for the field size of 2 × 2 cm<jats:sup>2</jats:sup> (compared to no B‐field case), which increased to 47.83% for the field size of 5 × 5 cm<jats:sup>2</jats:sup> in the presence of the magnetic field. Similarly, at the lung‐water interface, the dose decreased by 19.21% (relative to <jats:italic>D</jats:italic><jats:sub>max</jats:sub>) for a field size of 2 × 2 cm<jats:sup>2</jats:sup> and by 30.01% for 5 × 5 cm<jats:sup>2</jats:sup> field size. For more complex combinations of materials the dose deposition also becomes more complex.</jats:p></jats:sec><jats:sec><jats:title>Conclusions:</jats:title><jats:p>The GPUMCD algorithm showed good agreement against <jats:sc>geant4</jats:sc> both in the presence and absence of a 1.5 T external magnetic field. The application of 1.5 T magnetic field significantly alters the dose at the interfaces by either increasing or decreasing the dose depending upon the density of the material on either side of the interfaces.</jats:p></jats:sec>