Output correction factors in a 1.5 T magnetic field determined at the central axis and at the lateral dose maximum...

Objective.During commissioning of an MR-linac, field output factors are measured utilizing output correction factors,kB→,Qclin,Qmsrfclin,fmsr, which account for the influence of the field size and the magnetic field on the detector. In the literature to date,kB→,Qclin,Qmsrfclin,fmsrfor 1.5 T have only been determined for the central axis (CAX), including our recent work. At an MR-linac, the determination of the CAX position relies on e.g. MV-imaging and is at small field sizes located at the penumbra, whereas the maximum of the lateral profile (MAX) position is directly measurable. The aim of this study was to determine and comparekB→,Qclin,Qmsrfclin,fmsrat the CAX and MAX position.Approach.For this,kB→,QRatio-the influence of the magnetic field on the output correction factor without a magnetic field,kQclin,Qmsrfclin,fmsr-andkB→,Qclin,Qmsrfclin,fmsrwere determined for both CAX and MAX fully experimentally for two MR-optimized ionization chambers, their conventional counterparts, and a solid-state detector. Measurements were conducted using a water phantom in a mobile electromagnet positioned in front of a standard clinical linac (6 MV, 1.5 T). Profiles were measured with each detector to ensure accurate positioning at CAX/MAX. The uncertainty due to intra-type variation for tabulatedkB→,QRatioandkB→,Qclin,Qmsrfclin,fmsrwas assessed for the solid-state detector using four detectors. The change of absorbed dose to water was determined with alanine.Main results.Within the uncertainty, no difference inkB→,QRatiowas observed between the MR-optimized chambers and their conventional counterparts. For the solid-state detector, no difference between the CAX and MAX position was found. For all detectors,kB→,Qclin,Qmsrfclin,fmsrremained constant down to 2.5 × 2.5 cm². At smaller field sizes, for the MAX position and for all detectors,kB→,QRatiodecreased linearly. The reference point of the ionization chambers at the signal MAX position showed an average lateral displacement of 0.6 ± 0.1 mm over all field sizes compared to the dose MAX position, whereas the solid-state detector showed no displacement. The maximum standard uncertainty arising from intra-type variation was found to be 0.009 for the smallest field size, with and without a magnetic field.Significance.kB→,QRatioandkB→,Qclin,Qmsrfclin,fmsr, including its uncertainty, were determined for five detectors at CAX and-for the first time-MAX. For ionization chambers and small fields,kB→,Qclin,Qmsrfclin,fmsris lower at MAX than at CAX, whereas for the solid-state detector, it remains the same. For the uncertainty of tabulated correction factors, intra-type variation must be considered.