Would you mind asking your friend what machine was used for his radiation? Older machines have a 3-5 mm margin, and newer machines have 1-2 mm. The small margin means less chance the radiation will hit an organ at risk (in this case, the rectum). The machine for my radiation will be the Elekta MR-Linac (1-2 mm margin). Has any one in this support group had an experience with the MR-Linac (similar to MRidian).