Monte Carlo simulation of the effect of magnetic fields on brachytherapy dose distributions in lung tissue material

Abstract

The aim of this work was to use TOPAS Monte Carlo simulations to model the effect of magnetic fields on dose distributions in brachytherapy lung treatments, under ideal and clinical conditions. Idealistic studies were modeled consisting of either a monoenergetic electron source of 432 keV, or a polyenergetic electron source using the spectrum of secondary electrons produced by 192 Ir gamma-ray irradiation. The electron source was positioned in the center of a homogeneous, lung tissue phantom (ρ = 0.26 g/cm 3). Conversely, the clinical study was simulated using the VariSource VS2000 192 Ir source in a patient with a lung tumor. Three contoured volumes were considered: the tumor, the planning tumor volume (PTV), and the lung. In all studies, dose distributions were calculated in the presence or absence of a constant magnetic field of 3T. Also, TG-43 parameters were calculated for the VariSource and compared with published data from EGS-brachy (EGSnrc) and PENEL-OPE. The magnetic field affected the dose distributions in the idealistic studies. For the monoenergetic and poly-energetic studies, the radial distance of the 10% iso-dose line was reduced in the presence of the magnetic field by 64.9% and 24.6%, respectively. For the clinical study, the magnetic field caused differences of 10% on average in the patient dose distributions. Nevertheless, differences in dose-volume histograms were below 2%. Finally, for TG-43 parameters, the dose-rate constant from TOPAS differed by 0.09% ± 0.33% and 0.18% ± 0.33% with respect to EGS-brachy and PENELOPE, respectively. The geometry and anisotropy functions differed within 1.2% ± 1.1%, and within 0.0% ± 0.3%, respectively. The Lorentz forces inside a 3T magnetic resonance machine during 192 Ir brachytherapy treatment of the lung are not large enough to affect the tumor dose distributions significantly, as expected. Nevertheless, large local differences were found in the lung tissue. Applications of this effect are therefore limited by the fact that meaningful differences appeared only in regions containing air, which is not abundant inside the human.

Validation of HDR brachytherapy doses in the treatment of keloid scars using the egs_brachy Monte Carlo application

“…Although the planning with TG-43 dose model has been widely studied, its disadvantage is the two-dimensional calculation, dose-in-water approach and the lack of patient-specific modeling. Model-based dose calculation has its own guidelines, and RT can be planned more precisely with it (Beaulieu et al 2012, Duque et al 2020, Moreno-Barbosa et al 2020.…”

Effect of transverse magnetic field on dose distributions of yttrium 90 skin patch source

“…The effects of transverse and longitudinal magnetic fields on dose distributions in external pho-ton and electron beam treatments were investigated by different groups [7][8][9]. Moreno-Barbosa et al investigated the effects of magnetic fields on dose distributions using the TOPAS Monte Carlo (MC) code, in cases that underwent brachytherapy for lung tumors under clinical conditions [10]. C ¸avus ¸oglu et al investigated the behavior of beta particles in the magnetic field via GEANT4 MC simulation on the assumption that it would be possible to reduce the damage to normal tissues by using a magnetic field in cancer treatments using beta-emitting radioisotopes [11].…”

“…tissue, the mean path of the charged particles in the magnetic field gets the direction perpendicular to the magnetic field shorter and the number of interactions per unit time is increased [10]. Changes in the paths of the charged particles would trap the particles within the target tumor volume, providing an enhanced Bragg peak and highly localized strong dose accumulation within the tumor volume, thus reducing side effects and increasing therapeutic efficacy [14].…”