I had radiation to the rectum 2 years ago. The machine they used on me was a Varian TrueBeam, and it reduces collateral damage by shooting from multiple angles - I think 4 in my case. This allows the maximum dose to be delivered only at the intersection of the multiple beam angles. The other areas through which the x-rays travel get a partial dose that is much less damaging. The part that received the maximum dose (the rectum) was, of course, removed during the lower anterior resection (LAR) operation. They were very careful about targeting - using MRI and CT scans to model the exact position of the tumor and then tattooing dots on my skin to align the machine properly every time. I was impressed with the careful process and the level of technology applied to preventing collateral damage to healthy tissue.
Today I notice that when I have to pee I really have to pee - the sensation of having to go can be much more urgent than it used to be. I think that's a result of radiation. Although I don't think I have to go much more often. Sexual function has been ok but possibly affected somewhat.
All in all, I would take these relatively minor side effects any day over the significant risk of recurrence that I have hopefully reduced. So far I'm happy with the results.
Aug 2012: RC DX Stage IIIC, T3N2M0 by MRI - 38M
Fall 2012: Chemorad
Nov 2012: LAR - Path report: 0/13 LNs, tumor reduced to "microscopic foci"
Jan-Apr 2013: 8 rounds FOLFOX
May 2013: Ileostomy takedown, port removal
July 2013 (and since): NED!