Spot-wise compensation of proton beam deflection and radiation field deformation in the transverse magnetic field of an in-beam MR scanner using a research version of a commercial treatment planning system.

Objective
Magnetic resonance-integrated proton therapy (MRiPT) offers the potential for high-precision, image-guided particle therapy but is challenged by magnetic field (MF)-induced proton beam deflection and radiation field deformation. Unless compensated for, these effects compromise dose conformity and prohibit clinical implementation. This work aims to develop and experimentally validate a spot-wise compensation method capable of restoring proton dose distributions in the presence of a measured 3D magnetic vector field map within a clinical treatment planning system (TPS).
Approach
A spot-wise compensation framework was implemented in a research version of RayStation, a TPS commercially available for clinical use. The method compensates both lateral beam deflection and MF-induced radiation field deformation by shifting individual pencil-beam spots based on displacement vectors derived from dose calculations in water phantoms using the measured MF of an in-beam MR scanner integrated with a horizontal proton beamline. Two complementary strategies were implemented: a patient-specific approach based on plan-specific displacement calculations, and a generic approach using precomputed displacement maps derived from a reference phantom. Compensation performance was evaluated in representative clinical cases. Dose distributions were experimentally validated using 2D ionisation chamber array measurements.
Main Results
Both compensation methods effectively restored target coverage, dose homogeneity, and organ-at-risk sparing to the level of no-MF reference plans. The patient-specific method generally achieved a higher performance, whereas the generic compensation method enabled rapid automated correction with sufficient accuracy for clinical workflows involving deep-seated tumours. Local γ-analysis demonstrated excellent agreement between measured and calculated doses, with pass rates exceeding 98.5% for 3%/3 mm criteria.
Significance
This work demonstrates that compensation of MF-induced proton beam deflection and field deformation can be achieved entirely within a research version of the RayStation TPS using measured, MR scanner-specific MF data. The proposed method provides a scalable, robust framework for future MRiPT clinical applications.&#xD.