Development and Experimental Validation of an Energy Layer-wise Beam Deflection Compensation Method for MR-integrated Proton Therapy with a Transverse Magnetic Field.

Authors

K Godino Padre, F Lebbink, M Cobanaj, E Traneus, M Schürer, C Richter, D Kunath, J Hytry, S Menkel, H Fuchs, E G C Troost, J Pawelke, A Hoffmann, F Horst

Published in

International journal of radiation oncology, biology, physics. Aug 15, 2026. Epub Aug 15, 2026.

Abstract

Integrating magnetic resonance imaging with proton therapy (MRiPT) has the potential to improve its targeting accuracy. However, the magnetic field of the in-beam MR scanner causes proton beam deflection, distorting the dose distribution. In this work an energy-layer-wise beam deflection compensation method for MRiPT was developed and experimentally validated.
A research version of the RayStation treatment planning system (TPS) with a Monte Carlo dose calculation engine capable of including magnetic field effects was used to quantify lateral Bragg peak shifts in a water phantom geometry for energies of 100-220 MeV. Results were analyzed to create a parametrized, energy-dependent beam deflection compensation model that was integrated into the TPS. Three spread-out-Bragg-peak (SOBP) box fields and a patient treatment plan were optimized with deflections compensated per energy layer. Experimental validation was performed inside a 0.32 T in-beam MR scanner using three dosimetric setups: (1) an ionization chamber array with water-equivalent plates, (2) a water phantom with an ionization chamber, and (3) an anthropomorphic head phantom with radiochromic film inserts. Measurements were compared to calculated dose distributions via gamma analysis and for absolute dose and proton range agreement.
Gamma pass rates exceeded 97% for all SOBP box fields using 3%/3 mm criterion at 10% dose threshold. Absolute doses were accurate within 1.2%-2.2% depending on the energy range. Measured and calculated ranges agreed within 1.1 mm. The anthropomorphic head phantom experiment showed strong spatial agreement of calculated and measured dose distributions, achieving gamma pass rates >95% in all regions.
This work presents the first clinically suited and experimentally validated proton beam deflection compensation method for MRiPT integrated into a commercial TPS. The method efficiently and accurately restores dose distributions in the presence of the MR magnetic field and is ready to be applied for future MRiPT treatments.

PMID:
42603562
Bibliographic data and abstract were imported from PubMed on 16 Aug 2026.

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