title: Evaluation of a commercial MRI Linac based Monte Carlo dose calculation algorithm with GEANT4.
abstract: OBJECTIVE
  This paper provides a comparison between a fast, commercial, in-patient Monte Carlo dose calculation algorithm (GPUMCD) and geant4. It also evaluates the dosimetric impact of the application of an external 1.5 T magnetic field.
  
  
  METHODS
  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 GEANT4 (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(2). Dose scoring was performed using a 3D grid comprising 1 mm(3) 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.
  
  
  RESULTS
  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 Dmax), by both GPUMCD and GEANT4 for the field size of 2 × 2 cm(2) (compared to no B-field case), which increased to 47.83% for the field size of 5 × 5 cm(2) in the presence of the magnetic field. Similarly, at the lung-water interface, the dose decreased by 19.21% (relative to Dmax) for a field size of 2 × 2 cm(2) and by 30.01% for 5 × 5 cm(2) field size. For more complex combinations of materials the dose deposition also becomes more complex.
  
  
  CONCLUSIONS
  The GPUMCD algorithm showed good agreement against GEANT4 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.
authors: Sunnybrook Research Institute, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
authors: Syed Bilal
authors: SB
authors: Ahmad
authors: Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Department of Radiation Oncology, University of Toronto, 27 King's College Circle Toronto, Ontario M5S 1A1, Canada.
authors: Arman
authors: A
authors: Sarfehnia
authors: Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
authors: Moti Raj
authors: MR
authors: Paudel
authors: Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Department of Radiation Oncology, University of Toronto, 27 King's College Circle Toronto, Ontario M5S 1A1, Canada.
authors: Anthony
authors: A
authors: Kim
authors: Elekta, 13723 Riverport Drive, Suite 100, Maryland Heights, Missouri 63043.
authors: Sami
authors: S
authors: Hissoiny
authors: Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Department of Radiation Oncology, University of Toronto, 27 King's College Circle Toronto, Ontario M5S 1A1, Canada.
authors: Arjun
authors: A
authors: Sahgal
authors: Sunnybrook Health Sciences Center, Odette Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Department of Radiation Oncology, University of Toronto, 27 King's College Circle Toronto, Ontario M5S 1A1, Canada.
authors: Brian
authors: B
authors: Keller
mesh_terms: D000465:Algorithms; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D009010:Monte Carlo Method; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*