SU‐E‐J‐203: Investigation of 1.5T Magnetic Field Dose Effects On Organs of Different Density

Abstract

Purpose: For the combined 1.5T/6MV MRI‐linac system, the perpendicular magnetic field to the radiation beam results in altered radiation dose distributions. This Monte Carlo study investigates the change in dose at interfaces for common organs neighboring soft tissue. Methods: MCNP6 was used to simulate the effects of a 1.5T magnetic field when irradiating tissues with a 6 MV beam. The geometries used in this study were not necessarily anatomically representative in size in order to directly compare quantitative dose effects for each tissue at the same depths. For this purpose, a 512 cm3 cubic material was positioned at the center of a 2744 cm3 cubic soft tissue material phantom. The following tissue materials and their densities were used in this study: lung (0.296 g/cm3), fat (0.95), spinal cord (1.038), soft tissue (1.04), muscle (1.05), eye (1.076), trabecular bone (1.40), and cortical bone (1.85). Results: The addition of a 1.5T magnetic field caused dose changes of +46.5%, +2.4%, −0.9%, −0.8%, −1.5%, −6.5%, and −8.8% at the entrance interface between soft tissue and lung, fat, spinal cord, muscle, eye, trabecular bone, and cortical bone tissues respectively. Dose changes of −39.4%, −4.1%, −0.8%, −0.8%, +0.5%, +6.7%, and +10.9% were observed at the second interface between the same tissues respectively and soft tissue. On average, the build‐up distance was reduced by 0.6 cm, and a dose increase of 62.7% was observed at the exit interface between soft tissue and air of the entire phantom. Conclusion: The greatest changes in dose were observed at interfaces containing lung and bone tissues. Due to the prevalence and proximity of bony anatomy to soft tissues throughout the human body, these results encourage further examination of these tissues with anatomically representative geometries using multiple beam configurations for safe treatment using the MRI‐linac system. NSF GRFP Grant Award #LH‐102SPS

Low-density gel dosimeter for measurement of the electron return effect in an MR-linac

“…Our measured dose maps of the ERE are qualitatively consistent with previous phantom measurements and Monte Carlo simulations (Raaijmakers et al 2005, 2008, Lee et al 2015. A region of enhanced dose was observed in the higher density material just before the intersection with the lower density material, and a region of reduced dose was observed in the lower density material just before the boundary with the higher density material.…”

“…Two Monte Carlo studies that specifically looked at the lung-soft tissue interface also reported hot and cold spots of higher magnitude than measured in the current report. Raaijmakers et al (2008) and Lee et al (2015) measured hot spots of 49% and 46.5%, respectively, and cold spots of 36% and 39.4%, respectively, in Monte Carlo simulations with a 1.5 T field. The discrepancy between these results and the current study may be explained by the fact that the average CT numbers measured for the low-density gels are slightly higher than mean values for healthy lung tissue reported in the literature (typically around −700 to −800 HU) (Biernacki et al 1997, Shin et al 2011, Cressoni et al 2014, Ohkubo et al 2016.…”