Before using MR-images for radiotherapeutic treatment planning, it is necessary to check whether geometrical distortions influence the calculated dose distribution. In this study, 27 MRI units from different manufacturers and with different magnetic field strengths were investigated. First, all geometrical distortions and asymmetry were determined by specially defined parameters. Virtual planning target volume and organ of risk were drawn into MR images from different units and optimized dose distributions were calculated. The resulting field parameters were then transferred to the reference geometry. Geometric distortions caused small variations of the dose maximum (±0.5%). Twenty-five percent of the investigated calculated dose distributions showed an increase of the 95% isodose volume, 60% a decrease. The dose deviations in the organ of risk were comparable with the dose deviations in the planning target volume (PTV). This study has shown that measurements of geometrical distortions and asymmetry are necessary for MR-assisted treatment planning. For evaluation, we recommend dose calculations with hypothetical PTV's and organs of risk.