SU‐E‐J‐201: Investigation of MRI Guided Proton Therapy

Abstract

Purpose: Image‐guided radiation therapy has been employed for cancer treatment to improve the tumor localization accuracy. Radiation therapy with proton beams requires more on this accuracy because the proton beam has larger uncertainty and dramatic dose variation along the beam direction. Among all the image modalities, magnetic‐resonance image (MRI) is the best for soft tissue delineation and real time motion monitoring. In this work, we investigated the behavior of the proton beam in magnetic field with Monte Carlo simulations. Methods: A proton Monte Carlo platform, TOPAS, was used for this investigation. Dose calculations were performed with this platform in a 30cmx30cmx30cm water phantom for both pencil and broad proton beams with different energies (120, 150 and 180MeV) in different magnetic fields (0.5T, 1T and 3T). The isodose distributions, dose profiles in lateral and beam direction were evaluated. The shifts of the Bragg peak in different magnetic fields for different proton energies were compared and the magnetic field effects on the characters of the dose distribution were analyzed. Results: Significant effects of magnetic field have been observed on the proton beam dose distributions, especially for magnetic field of 1T and up. The effects are more significant for higher energy proton beam because higher energy protons travel longer distance in the magnetic field. The Bragg peak shift in the lateral direction is about 38mm for 180MeV and 11mm for 120MeV proton beams in 3T magnetic field. The peak positions are retracted back for 6mm and 2mm, respectively. The effect on the beam penumbra and dose falloff at the distal edge of the Bragg peak is negligible. Conclusion: Though significant magnetic effects on dose distribution have been observed for proton beams, MRI guided proton therapy is feasible because the magnetic effects on dose is predictable and can be considered in patient dose calculation.

Prediction and compensation of magnetic beam deflection in MR-integrated proton therapy: a method optimized regarding accuracy, versatility and speed

“…The lateral deflection ∆y and longitudinal retraction ∆x were calculated as functions of E 0 and B 0 with both the new model and the two analytical models (AI and TG, as discussed in section 2). Currently being the most accurate method for proton trajectory prediction, published values obtained by Monte Carlo particle tracking (Raaymakers et al 2008, Moteabbed et al 2014, Li 2015, Moser 2015) were compared to the results gained with the three models. Correction parameters ∆γ and ∆E 0 were calculated and compared to the TG method, and beam trajectories obtained with both correction methods were reconstructed in order to evaluate whether a distance remains to the intended Bragg peak position.…”

“…So far, MR-integrated proton therapy (MRiPT) has only been described as a hypothetical modality in simulation studies (Raaymakers et al 2008, Wolf & Bortfeld 2012, Moteabbed et al 2014, Oborn et al 2015, Li 2015, Hartman et al 2015, Moser 2015. Likewise challenging as in MR-integrated photon therapy, there are specific technological and physical problems that need to be solved when facing MRiPT.…”

“…Probably more problematic is the fact that, being charged particles, the therapeutic protons will be deflected by the magnetic field of the MRI scanner (Oborn et al 2016). Different studies have focussed on quantifying the beam deflection effect for dosimetric and treatment planning purposes using particle tracking with Monte Carlo simulations (Raaymakers et al 2008, Moteabbed et al 2014, Oborn et al 2015, Li 2015, Moser 2015 or analytical models (Wolf & Bortfeld 2012, Hartman et al 2015.…”

Modulation of lateral positions of Bragg peaks via magnetic fields inside cancer patients: Toward magnetic field modulated proton therapy

“…Several beneficial studies are available on the magnetic‐induced dose distortion and corresponding modification methods in the presence of magnetic fields during magnetic resonance imaging‐guided proton therapy. In these studies, the magnetic‐induced proton deflections inside patients were considered as adverse factors when distorting well‐designed dose distributions in RTPs. Unlike in previous studies, the magnetic‐induced deflections inside patients are deemed as advantageous influencing factors for reducing vital organ doses without sacrificing tumor dose coverage in this study.…”