Dosimetry in 1.5 T MR-Linacs: Monte Carlo determination of magnetic field correction factors and investigation of the air gap effect.

Background: In Magnetic Resonance-Linac (MR-Linac) dosimetry formalisms, a new correction factor, k , has been introduced to account for corresponding changes to detector readings under the beam quality, Q, and the presence of magnetic field, B.

Purpose: This study aims to develop and implement a Monte Carlo (MC)-based framework for the determination of k correction factors for a series of ionization chambers utilized for dosimetry protocols and dosimetric quality assurance checks in clinical 1.5 T MR-Linacs. Their dependencies on irradiation setup conditions are also investigated. Moreover, to evaluate the suitability of solid phantoms for dosimetry checks and end-to-end tests, changes to the detector readings due to the presence of small asymmetrical air gaps around the detector's tip are quantified.

Methods: Phase space files for three irradiation fields of the ELEKTA Unity 1.5 T/7 MV flattening-filter-free MR-Linac were provided by the manufacturer and used as source models throughout this study. Twelve ionization chambers (three farmer-type and nine small-cavity detectors, from three manufacturers) were modeled (including their dead volume) using the EGSnrc MC code package. k values were calculated for the 10 × 10 cm irradiation field and for four cardinal orientations of the detectors' axes with respect to the 1.5 T magnetic field. Potential dependencies of k values with respect to field size, depth, and phantom material were investigated by performing additional simulations. Changes to the detectors' readings due to the presence of small asymmetrical air gaps (0.1 up to 1 mm) around the chambers' sensitive volume in an RW3 solid phantom were quantified for three small-cavity chambers and two orientations.

Results: For both parallel (to the magnetic field) orientations, k values were found close to unity. The maximum correction needed was 1.1%. For each detector studied, the k values calculated for the two parallel orientations agreed within uncertainties. Larger corrections (up to 5%) were calculated when the detectors were oriented perpendicularly to the magnetic field. Results were compared with corresponding ones found in the literature, wherever available. No considerable dependence of k with respect to field size (down to 3 × 3 cm ), depth, or phantom material was noticed, for the detectors investigated. As compared to the perpendicular one, in the parallel to the magnetic field orientation, the air gap effect is minimized but is still considerable even for the smallest air gap considered (0.1 mm).

Conclusion: For the 10 × 10 cm field, magnetic field correction factors for 12 ionization chambers and four orientations were determined. For each detector, the k value may be also applied for dosimetry procedures under different irradiation parameters provided that the orientation is taken into account. Moreover, if solid phantoms are used, even the smallest asymmetrical air gap may still bias small-cavity chamber response. This work substantially expands the availability and applicability of k correction factors that are detector- and orientation-specific, enabling more options in MR-Linac dosimetry checks, end-to-end tests, and quality assurance protocols.