Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom

Abstract

Purpose: Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Methods: Two cylindrical electromagnets were used to produce a 0.207 T magnetic field parallel to the central axis of a 6 MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. Results: The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207 T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2 mm c acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field D max ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (> 0.13 T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Conclusions: Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately ...

Technical Note: Experimental verification ofEGSnrc calculated depth dose within a parallel magnetic field in a lung phantom

“…Shortening the step size in this manner has also been shown to result in magnetic field dose simulations more accurately matching our polystyrene measurements . A previously benchmarked phase space file represented the 6 MV beam of the Varian Silhouette unit and was used as the source in the DOSXYZnrc simulations. COMSOL Multiphysics V4.4 (Burlington, MA) calculated the 3D magnetic field map that was benchmarked against measurements as previously described …”

“…With the reduced step size and using the default condensed history technique, this implementation is expected to pass the Fano cavity test as it is similar to one of the cases investigated by Malkov and Rogers . Shortening the step size in this manner has also been shown to result in magnetic field dose simulations more accurately matching our polystyrene measurements . A previously benchmarked phase space file represented the 6 MV beam of the Varian Silhouette unit and was used as the source in the DOSXYZnrc simulations.…”

“…For the Monte Carlo simulations, DOSXYZnrc was slightly modified to read the 3D magnetic field. The standard magnetic field implementation, based on the work by A. F. Bielajew, already present in the macro package “emf_macros”, was used but with a shorter particle step size.…”

“…This can cause a substantial dose increase at tissue — air or tissue — lung interfaces, that has been termed as the electron return effect (ERE) . Systems with the radiation beam central axis parallel to the main magnetic field have a minimal dose increase at tissue — air surfaces . However, the parallel magnetic field is preventing the contaminant electrons accompanying the photon beam from scattering laterally, causing an entrance surface dose increase that can be substantial, measurable, or minimal depending on the MRI's fringe magnetic field and design.…”

“…All these dose modifications have previously been investigated using several Monte Carlo (MC) packages: Geant4, EGSnrc, and PENELOPE . While Geant4 dose calculations in the presence of a magnetic field have been previously verified experimentally for multiple materials, to date, EGSnrc MC has only been recently experimentally verified in polystyrene . The current study aims to extend the experimental verification of EGSnrc with a magnetic field parallel to the radiation beam central axis to lung‐like materials as well.…”