Charged particle transport in magnetic fields in EGSnrc

• Published in: Medical physics (Lancaster) Vol. 43; no. 7; pp. 4447 - 4458
• Main Authors: Malkov, V. N., Rogers, D. W. O.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01.07.2016
• Subjects: 60 APPLIED LIFE SCIENCES; Algorithms; Biological material, e.g. blood, urine; Haemocytometers; biomedical MRI; Cavitation; CHARGED-PARTICLE TRANSPORT; Computer Simulation; COMPUTERIZED SIMULATION; Dosimetry; EGSnrc; Electron sources; Fano test; Gas‐filled counters: ionization chambers, proportional, and avalanche counters; High field transport; Involving electronic [emr] or nuclear [nmr] magnetic resonance, e.g. magnetic resonance imaging; Ion beams; ion chamber; ionisation chambers; IONIZATION CHAMBERS; Magnetic effects; magnetic field; Magnetic field sensors; Magnetic Fields; Magnetic resonance imaging; Measurement of nuclear or x‐radiation; Models, Theoretical; Monte Carlo; MONTE CARLO METHOD; Monte Carlo methods; Monte Carlo simulations; Motion; PHANTOMS; Phantoms, Imaging; PHOTON BEAMS; Photons; RADIATION DOSES; RADIATION PROTECTION AND DOSIMETRY; radiation therapy; Radiation, Ionizing; Therapeutic applications, including brachytherapy; Tubes for determining the presence, intensity, density or energy of radiation or particles; Water; with scintillation detectors; EGSnrc; magnetic field; ion chamber; Monte Carlo; Fano test
• ISSN: 0094-2405; 2473-4209; 1522-8541; 2473-4209
• DOI: 10.1118/1.4954318

Purpose: To accurately and efficiently implement charged particle transport in a magnetic field in EGSnrc and validate the code for the use in phantom and ion chamber simulations. Methods: The effect of the magnetic field on the particle motion and position is determined using one- and three-point numerical integrations of the Lorentz force on the charged particle and is added to the condensed history calculation performed by the EGSnrc PRESTA-II algorithm. The code is tested with a Fano test adapted for the presence of magnetic fields. The code is compatible with all EGSnrc based applications, including egs++. Ion chamber calculations are compared to experimental measurements and the effect of the code on the efficiency and timing is determined. Results: Agreement with the Fano test’s theoretical value is obtained at the 0.1% level for large step-sizes and in magnetic fields as strong as 5 T. The NE2571 dose calculations achieve agreement with the experiment within 0.5% up to 1 T beyond which deviations up to 1.2% are observed. Uniform air gaps of 0.5 and 1 mm and a misalignment of the incoming photon beam with the magnetic field are found to produce variations in the normalized dose on the order of 1%. These findings necessitate a clear definition of all experimental conditions to allow for accurate Monte Carlo simulations. It is found that ion chamber simulation times are increased by only 38%, and a 10 × 10 × 6 cm3 water phantom with (3 mm)3 voxels experiences a 48% increase in simulation time as compared to the default EGSnrc with no magnetic field. Conclusions: The incorporation of the effect of the magnetic fields in EGSnrc provides the capability to calculate high accuracy ion chamber and phantom doses for the use in MRI-radiation systems. Further, the effect of apparently insignificant experimental details is found to be accentuated by the presence of the magnetic field.