The geometrical distortions of MR images are potential source of error in MR-based radiation therapy planning (RTP) which requires accurate anatomical delineation. Beside the system-specific residual distortions, presence of any susceptibility-mismatch within the region of interest may lead to image distortion. We have recently proposed an algorithm based on quantitative susceptibility mapping (QSM) for post-implant dosimetry of prostate brachytherapy seeds. In this study, the undistorted field map in patients with and without implanted seeds was estimated and images were corrected accordingly, then QSM was performed. In patients with implanted seeds, distortion correction improved the accuracy of the QSM reconstruction.