Dosimetric Changes of Magnetic Control System to Improve Anatomy in Abdominal SBRT

Abstract

Purpose: Radiation induced toxicities in bowel remains a challenge for SBRT. Due to sharp dose falloffs, small displacements of the bowel can lead to meaningful dose reduction. Magnetic actuation systems (MAS) have been developed for varying clinical applications (surgery, endoscopy, etc.), but this technology has not yet been applied to radiotherapy. These systems may hold promise for use in radiotherapy, giving the ability to displace the bowel with exogenous magnetic fields. Given that high-Z magnetic material and magnetic fields near treatment fields can perturb radiation dose deposition, in this specific study we quantify dosimetric changes due to presence of this apparatus.Methods: First, magnetic hydrogels were investigated for use in radiotherapy. Previously investigated magnetic hydrogels were recreated and dosimetric measurements were made. X-ray radiographs of the magnetic hydrogel were taken with on-board imaging of a linear accelerator (LINAC) on anthropomorphic phantoms and with a CT in an electron density phantom. Additionally, attenuation properties and lateral scattering of magnetic hydrogels within a 6 MV photon beam were measured. Secondly, a prototype MAS was modeled in TOPAS MC (v3.9), a Monte Carlo software that rigorously tracts radiation transport, and radiation dose effects were quantified. The simulation was first verified against measured data of a Varian TrueBeam LINAC. Magnetic fields, generated through COMSOL Multiphysics (v5.4), and internal magnetic capsule design of the MAS were included in the simulation. A 6X field (4x4cm2) was incident on a 150 mm water cube (1mm voxels, 800M histories). A variety of relevant scenarios were simulated: moving a hypothetical bowel 1 cm away from field, the internal magnet within the field, an air cavity outside the field, and an air cavity extending into the field. Results were compared to simulations with none of the MAS properties included. Results: Magnetic hydrogels showed contrast to other high attenuating material on a pelvic, thorax, and electron density phantom. The magnetic hydrogel maxed out the HU on the CT scanner and caused large streaking artifacts. Lateral scattering due to the magnetic hydrogel had a maximum increase 8.5% and dose behind the magnetic hydrogel maximum decrease of 11%. The Monte Carlo simulation gave good agreement with measured LINAC data (RMS of difference between percent depth dose <1%). When the hypothetical bowel was moved 1 cm away from field, max and mean dose to the bowel decreased by 4.02 and 2.69 times respectively. When the magnet was in field, the backscatter from the magnet increased dose by 33.9%±0.9% at the interface but dropped to 1% by 5mm. Significant dose reduction occurred directly behind the magnet (-29.6%±0.7%). Simulations of the magnetic hydrogel within the field gave a dose increase of 9.5±0.8% upstream from the hydrogel and a decrease of -10.7±0.6% downstream. No significant effect was seen when an air cavity was present outside or within the field. Conclusion: The proposed method of displacing the bowel with a MAS has potential to significantly reduce dose to proximal regions of the bowel, however local dose enhancement must be managed via a low Z coating. A magnetic hydrogel is expected to have a smaller dose effects than a magnetic capsule, but they are not currently being used in conjunction with MASs. The data suggests that magnetic fields from MAS have minimal impact on dose deposition due to lack of hot or cold spots within the air cavities.