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Magnetic resonance image-guided radiotherapy (MRgRT) has the potential to increase the accuracy of radiation treatment delivery. Several research groups have developed hybrid MRgRT devices differing by radiation source used and magnetic field orientation and strength. In this work, we investigate the impact of different magnetic field orientations and strengths on the treatment planning of nonsmall cell lung cancer patients (NSCLC).A framework using the in-house developed treatment planning system matRad and the EGSnrc Monte Carlo code system was introduced to perform Monte Carlo-based treatment planning in the presence of a magnetic field. A specialized spectrum-based source model for the beam qualities of 6 MV and cobalt-60 was applied. Optimized plans for stereotactic body radiation therapy (SBRT) and intensity-modulated radiation therapy (IMRT) were generated for four NSCLC patients in the presence of different magnetic field orientations and strengths which are applied in hybrid MRgRT devices currently under development or in clinical use.SBRT and IMRT treatment planning could be performed with consistent plan quality for all magnetic field setups. Only minor effects on the treatment planning outcome were found in the case of magnetic fields orientated perpendicular to the beam direction. Compared to the perpendicular magnetic field orientation, the inline orientation showed the capability to reduce the dose to lung while maintaining equal target coverage. Particularly for tumors with a central position in lung, a distinct dose reduction was obtained which led to a maximum reduction of mean lung dose by 18.5% (0.5 Gy), when applying a 1 T inline magnetic field.All plans generated in this work obtained dose metrics within clinical constraints according to RTOG guidelines. When considering conventional dose metrics, no detrimental effects due to the magnetic fields were observed on the dose to the tumor or to organs at risk. An evaluation of the effects on skin dose was not ascertainable due to the simplified specification of the source model used. By accounting for the magnetic field during treatment planning, a dose reduction in lung could be achieved for inline-oriented magnetic fields. An inline orientation of the magnetic field therefore showed a potential benefit when treating NSCLC with MRgRT.