Integrated MRI-guided proton therapy planning: Accounting for the full MRI field in a perpendicular system

Purpose: To present a first study on the treatment planning feasibility in perpendicular field MRI-integrated proton therapy that considers the full transport of protons from the pencil beam scanning (PBS) assembly to the patient inside the MRI scanner. Methods: A generic proton PBS gantry was modeled as being integrated with a realistic split-bore MRI system in the perpendicular orientation. MRI field strengths were modeled as 0.5, 1, and 1.5 T. The PBS beam delivery and dose calculation was modeled using the TOPAS Monte Carlo toolkit coupled with matRad as the optimizer engine. A water phantom, liver, and prostate plans were evaluated and optimized in the presence of the full MRI field distribution. A simple combination of gantry angle offset and small PBS nozzle skew was used to direct the proton beams along a path that closely follows the reference planning scenario, that is, without magnetic field. Results: All planning metrics could be successfully achieved with the inclusion of gantry angle offsets in the range of 8 (Formula presented.) –29 (Formula presented.) when coupled with a PBS nozzle skew of 1.6 (Formula presented.) –4.4 (Formula presented.). These two hardware-based corrections were selected to minimize the average Euclidean distance (AED) in the beam path enabling the proton beams to travel inside the patient in a path that is close to the original path (AED smaller than 3 mm at 1.5 T). Final dose optimization, performed through further changes in the PBS delivery, was then shown to be feasible for our selection of plans studied yielding comparable plan quality metrics to reference conditions. Conclusions: For the first time, we have shown a robust method to account for the full proton beam deflection in a perpendicular orientation MRI-integrated proton therapy. These results support the ongoing development of the current prototype systems.