Author: Justus Adamson, Wesley Andrew Cunningham, Chang-Sei Kim, Yeongoh Ko, Anna E. Rodrigues 👨🔬
Affiliation: Duke University, Chonnam National University, Duke University Medical Center 🌍
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. Remote magnetic navigation systems (RMNS) have been developed for magnetic capsule endoscopy; these 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: TOPAS MC (v3.9) was used to simulate radiation dose for a 6X field (4x4cm2) incident on a 150 mm water cube (1mm voxels, 800M histories). Magnetic fields generated from a RMNS consisting of 4 coils (627 turns of wire per coil, maximum current: 10A, maximum magnetic field strength: 256mT) and magnet capsule (9mmx9mmx18mm neodymium) were simulated via COMSOL Multiphysics (v5.4) and imported into TOPAS. The capsule was modeled both within and 25mm away from the field. Dose differences were determined between simulations in water, with and without the coil/magnet apparatus. A hypothetical small bowel region was defined with its edge placed at the field edge in the water simulation and 1 cm away from field edge in the full apparatus simulation.
Results: 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%). Max and mean dose to the bowel decreased by 4.02 and 2.69 times respectively with the magnet moving the bowel 1 cm away from the field edge.
Conclusion: The proposed method of displacing the bowel with a RMNS has potential to significantly reduce dose to proximal regions of the bowel, however local dose enhancement must be managed via a low Z coating.