Determination of output correction factors in magnetic fields using two methods for two detectors at the central axis

Objective.Commissioning an MR-linac treatment planning system requires output correction factors,kB→,Qclin,Qmsrfclin,fmsr, for detectors to accurately measure the linac's output at various field sizes. In this study,kB→,Qclin,Qmsrfclin,fmsrwas determined at the central axis using two methods: one that combines the corrections for the influence of the magnetic field and the small field in a single factor (kB→,Qclin,Qmsrfclin,fmsr), and a second that isolates the magnetic field's influence, allowing the use of output correction factors without a magnetic field,kQclin,Qmsrfclin,fmsr, from literature for determiningkB→,Qclin,Qmsrfclin,fmsr.Approach.To determinekB→,Qclin,Qmsrfclin,fmsrand examine its behaviour across different photon energies and magnetic flux densitiesBin small fields, measurements with an ionization chamber (0.07 cm³sensitive volume) and a solid-state detector were carried out at an experimental facility for both approaches. Changes in absorbed dose to water with field size were determined via Monte Carlo simulations. To evaluate clinical applicability, additional measurements were conducted on a 1.5 T MR-linac.Main results.Both methods determined comparablekB→,Qclin,Qmsrfclin,fmsrresults. For field sizes >3 × 3 cm²,Branging from -1.5 to 1.5 T and photon energies of 6 and 8 MV, no change ofkQclin,Qmsrfclin,fmsras a function of the magnetic field was observed. Comparison with measurement results from the 1.5 T MR-linac confirm this. For ⩽3 × 3 cm²,kB→,Qclin,Qmsrfclin,fmsrdepends on photon energy andB. For 1.5 T and 6 MV,BreduceskQclin,Qmsrfclin,fmsrup to 3% for the ionization chamber and up to 7% for the solid-state detector.Significance.kB→,Qclin,Qmsrfclin,fmsrwere successfully determined for two detectors, enabling their use at a 1.5 T MR-linac. For field sizes of >3 × 3 cm²,kB→,Qclin,Qmsrfclin,fmsris one for most detectors suitable for small field dosimetry for all available perpendicular MR-linac systems, as confirmed in the literature. For these field sizes and detectors, the correction factor accounting for the dosimeter response change in the reference field due to the magnetic field,kB→,Qmsrfmsr, can be used for cross-calibration. Therefore, future research may only focus on small field sizes.