Impact of magnetic fields on small field dosimetry for parallel MR‐linacs

Abstract Background The use of MR‐guided radiation therapy (MRgRT) is increasing with the introduction of commercially‐available parallel MR‐linac systems. Treatment fields used by these machines can be very small and highly modulated. Literature on the small field dosimetry of parallel MR‐linacs and the impact of the magnetic field on dose perturbations is scarce. Purpose To investigate the impact of magnetic fields on small field dosimetry of parallel MR‐linacs using Monte Carlo (MC) methods. Methods A TOPAS MC model of a 6 MV FFF beam was developed and validated against a commercial 0.5 T parallel MR‐linac. The impact of varying magnetic field of up to 3.0 T on water phantom dosimetry such as percent depth dose (PDD), beam profiles, and output factors (OFs) was studied. A lung phantom with embedded spherical tumors of diameters 1–3 cm was employed to investigate the impact of parallel magnetic fields on lung lesions using an arc delivery with a 4 ο gantry spacing providing a surrogate for rotational delivery techniques. Dose distributions were compared with the 0.0 T scenario. Results Negligible differences were noted in the PDD, except for the build‐up region, between the investigated parallel magnetic fields. Besides the tail region of the 1.5 and 3.0 T profiles, minimal differences were observed in the lateral beam profiles. Similarly, parallel magnetic fields of up to 1 T had a negligible impact on small field OFs. Compared to the 0 T magnetic field, the OF for field sizes of <2 × 2 cm 2 was found to increase significantly, by up to 9%, for the 1.5 and 3.0 T fields. The increase in magnetic field strength led to a more uniform dose distribution across the tumor inside the lung phantom with a slight reduction in penumbra due to the electron focusing effect. Normalized to the 0 T lung phantom dose distribution, differences ranging from 3% to 8% were found when the magnetic field strength was varied from 0.5 to 3.0 T. These differences were proportional to the field strength and were more significant for the smaller tumor diameters employing smaller fields. Conclusions The impact of parallel magnetic fields of up to 1.0 T strength is minimal for in‐water small field dosimetry. However, the increase in OF, compared to the 0 T case, was found to be significant for the 3.0 T magnetic field. For all field strengths compared to 0 T, significant dose differences were found around the periphery of the tumors inside a lung phantom that must be accounted for during treatment planning and dosimetry.