I had 20 rounds of SBRT. While I didn't have diarrhea, there was radiation damage visible near the end of my digestive tract during my last coloniscopy (2 years after the radiation). I also took damage to the bottom of my bladder (confirmed by cystoscopy).

It's not just a matter of how accurate the radiation beam is, though SBRT is a big improvement over earlier lead-blanket approaches (with the narrow beam annd constant rotation, superficial burns to skin are now very rare, for example).

My radiation oncologist explained to me that the radiation introduced into the prostate continues to kill cells — cancerous or otherwise — for weeks after the treatments finish, so I wouldn't be surprised if it can spread out a bit from your prostate to surrounding organs (just a layperson's guess).

That can also be a good thing, because it might attack any undetected localised cancer spread (which surgery doesn't), but it can still create discomfort.