GTV contouring variability was seen between T1c, T2f and T2 sequences. The coincidence of T1+c and T2f was better than T1+c and T2 in BMs. Since reductions in volume and changes of lesion center was observed during HFRT, the use of MR-guided radiation therapy (RT) and treatment adaptation is needed. The optimal timing for treatment plan modification might be when the course of treatment reaches 2/3 for most large BMs. Further research to find out patients who may benefit form MR-guided adaptive RT is ongoing.