My RO told me that the normal procedure is to cover the whole prostate and that the millimeters beyond that can affect healthy tissue. Picking the radiation machines that limits those millimeters made sense to me. Some machines set margins greater than 4 mm, the MRIdian margins were set to 2 mm for me. MRI shows a clearer picture of healthy tissue, CT less so. I am not a dr but that’s what I have heard and so far my side effects are minimal from the MRIdian.