Modeling the Potential Use of an External Magnetic Navigation System In Abdominal SBRT
Abstract
Purpose
Radiation induced bowel toxicity is challenging for abdominal SBRT due to proximity to PTV and high dose/fraction. Remote magnetic navigation systems (RMNS) have been developed for magnetic capsule endoscopy, in which the RMNS mounted on a robotic arm controls the location and orientation of a magnetic capsule within the bowel. Our goal is to apply this technology to enable subtle shifts in bowel location relative to PTV to achieve more favorable anatomy and reduce SBRT radiation dose. In this work we develop a mathematical framework for the behavior of gastrointestinal (GI) tract and surrounding abdomen under magnetic forces.
Methods
Two mathematical models were developed to predict behavior of the capsule in the bowel; both portrayed bowel and surrounding tissue as a double spring-based system with spring constant determined via Young’s Modulus of tissue. The first used springs in series and utilized Euler-Bernoulli Beam Theory; the second used springs in parallel to model a capsule fixed within abdominal cavity. The models were applied to calculate the bowel displacement achievable from a prototype magnetic capsule (neodymium, 9mm-diameter, 18mm-length) and RMNS (4 coils, 571 loops, 10A) mounted on a robotic arm. Potential dose sparing was quantified for prior clinical SBRT cases (n=3) identified as potential candidates for RMNS intervention.
Results
Modeling indicated that the prototype RMNS could displace the capsule (and bowel proximal to the PTV) by 0.5cm, 1.0cm, 1.5cm and 2.0cm at depths within the patient of 8.8-8.9cm, 5.3-6.5cm, 3.6-5.4cm, and 2.0-4.8cm, respectfully. For a prior adrenal SBRT case with a capsule placed at bowel edge proximal to PTV (9.1cm depth), the potential RMNS displacement was 0.4- 0.5cm which decreased max bowel dose from 99.2% of Rx to 74.8-78.9%.
Conclusion
Mathematical modeling indicates the potential clinical benefit of using an RMNS to displace bowel away from the PTV for abdominal SBRT.