A fast analytical dose calculation approach for MRI-guided proton therapy

Abstract

Objective. Magnetic resonance (MR) is an innovative technology for online image guidance in conventional radiotherapy and is also starting to be considered for proton therapy as well. For MR-guided therapy, particularly for online plan adaptations, fast dose calculation is essential. Monte Carlo (MC) simulations, however, which are considered the gold standard for proton dose calculations, are very time-consuming. To address the need for an efficient dose calculation approach for MRI-guided proton therapy, we have developed a fast GPU-based modification of an analytical dose calculation algorithm incorporating beam deflections caused by magnetic fields. Approach. Proton beams (70–229 MeV) in orthogonal magnetic fields (0.5/1.5 T) were simulated using TOPAS-MC and central beam trajectories were extracted to generate look-up tables (LUTs) of incremental rotation angles as a function of water-equivalent depth. Beam trajectories are then reconstructed using these LUTs for the modified ray casting dose calculation. The algorithm was validated against MC in water, different materials and for four example patient cases, whereby it has also been fully incorporated into a treatment plan optimisation regime. Main results. Excellent agreement between analytical and MC dose distributions could be observed with sub-millimetre range deviations and differences in lateral shifts <2 mm even for high densities (1000 HU). 2%/2 mm gamma pass rates were comparable to the 0 T scenario and above 94.5% apart for the lung case. Further, comparable treatment plan quality could be achieved regardless of magnetic field strength. Significance. A new method for accurate and fast proton dose calculation in magnetic fields has been developed and successfully implemented for treatment plan optimisation.

The impact of motion on onboard MRI-guided pencil beam scanned proton therapy treatments

“…These dose distributions illustrate the impact of different magnetic field strengths and gantry angle adjustments. As discussed in Duetschler et al (2023b), a similar static plan quality is achieved independent of the magnetic field strength. Moreover, a similar plan quality is also achieved when altering the gantry angle to emulate the beam trajectory without magnetic field influence.…”

“…The analytical dose calculation algorithm accounting for the deflection of the proton beams in a perpendicular homogenous magnetic field is described in detail in Duetschler et al (2023b) and we only provide a short description here. The dose calculation is based on ray casting (Schaffner et al 1999), which has been adjusted to account for the deflected beam trajectories.…”

“…These LUTs were generated from TOPAS MC (Perl et al 2012, Faddegon et al 2020 calculations of single pencil beams (70-229 MeV, 115 energies) in water in orthogonal magnetic fields (0.5/1.5 T). A validation of the algorithm against MC in different media and for patient cases is also presented in Duetschler et al (2023b).…”

“…As the particles are charged, however, the effect of the magnetic field on proton beam dose deposition has to be considered during the dose calculation and treatment planning. This has been investigated in various simulation studies using Monte Carlo (MC) simulations (Raaymakers et al 2008, Moteabbed et al 2014, Hartman et al 2015, Oborn et al 2015, Fuchs et al 2017, Kurz et al 2017, Burigo and Oborn 2019, Lühr et al 2019, Santos et al 2019, Burigo and Oborn 2021), while analytical or numerical approaches to account for the deflection of proton beams in a perpendicular magnetic field have also been proposed (Wolf and Bortfeld 2012, Hartman et al 2015, Fuchs et al 2017, Schellhammer and Hoffmann 2017) and integrated into analytical dose calculation algorithms by Padilla-Cabal et al (2018, 2020, Duetschler et al (2023b). Furthermore, results from the operation of experimental systems have been reported (Schellhammer et al 2018a, 2018b, Lühr et al 2019, Padilla-Cabal et al 2019, Gantz et al 2020, 2021, Fuchs et al 2022.…”

“…Thus, for the future implementation of online MRI-guided proton therapy for moving tumours, the dose calculation should not only consider the impact of the magnetic field on the dose distributions, but also the effect of motion on the dose. Therefore, in this paper, a fast MR-compatible analytical dose calculation, as developed by Duetschler et al (2023b), has been extended to 4D dose calculations and used to perform, to our knowledge, first dosimetric simulations that consider the joint impact of respiratory motion and magnetic fields on PBS proton therapy. As such, and as a 'worst case' scenario, we investigate here the potential effect of unmitigated motions on MR-guided PBS plans, as well as the potential effectiveness of rescanning (arguably the simplest motion management technique) on such treatments.…”