In the U.S., they usually administer SBRT as a short series of 5 high radiation doses, partly because Medicare won't pay for more than 5 sessions of it (at least, that's what I read), while they usually administer IMRT as a longer series of smaller doses.

I'm guessing that might be part of why your doctor is recommending IMRT — they don't want your individual doses to be too big

Here in Canada, I had my SBRT fractionated into 20 lower-dose sessions, just like with IMRT, so it wasn't an issue.

There is no reason you cannot get a second opinion from another radiation oncologist. I’m not sure if you are at a center of excellence, but that might be the best place to go to get that second opinion.

I had seven weeks of IMRT and had absolutely no reaction to it at any time while it was going on. Five years later, I started to have incontinence problems, But until that I had no side effects from the radiation at all. Some people get fatigue, others get urinary issues requiring Flomax and infrequently something else.

As a layman, I think the benefit of IMRT is that the organs at risk will be "better able to self-heal" after small dose radiation exposures over 28 treatment days. I had the 5-faction high dose SBRT; I noticed a little blood in my stool for a few days post-treatment; someone in this forum opined that my rectal area was healing/the veins were still sensitive, so some blood got picked by the bowel on the way out. Perhaps you will not experience this "little bleeding' post-IMRT -- the only side effect that got me worried a little bit.