Hey Ken, I had the same question for my RO before salvage radiation (no prostate). He said you can have cancer cells close - but not on or attached - to the rectum; the radiation cannot get to it with a gel spacer in place.
You will have a simulation first and you will be required to take an enema before the procedure. This ensures an empty, smaller diameter rectum which will be digitally scanned into their treatment parameters. Also NO gassy foods 2-3 days before!
In short, the RO can “shape” the beam around the rectum to avoid damaging it while still hitting wayward cells. It is CRITICAL that your no gas/low fermentable fiber diet be strictly adhered to, otherwise your rectal diameter might be slightly larger during treatment and the pre-programmed radiation beams could hit it, causing damage. But the techs should see this when they scan you (every day) prior to initiating treatment.
Be sure to speak to your team about all this because you want to avoid any problems down the road! Best
Phil

Space oats worked fine for me. My gleasons scores were 8,9,10. Had seminal invasion too. I guess it’s where the location of the cancer is on the prostate. Don’t remember any specific diet to follow. It was a piece of cake for me , of course going thru the perineum. My Fanny works fine using stool softeners. This event was back in 2022

In similar circumstances, my RO also considered doing without the SpaceOAR. He then learned I had a history of hemorrhoids and proceeded with the SpaceOAR, explaining that he could work around it and get the radiation to where it needed to be.

Are there any contraindications to doing an enema three weeks after fiducial markers and Barrigel placements to "ensure" a clean rectum for EBRT ?

If he has concerns because of the amount of cancer you have in that area of your prostate (that he can’t use SpaceOAR or Barrigel), then he should consider using the newer product called BioProtect.

And if he won’t try any of those, there is still the option of the old-fashion rectal spacer - called an endorectal balloon - that can be used.

I'm scheduled for a prostatectomy next month. When I met with the RO, she said she never uses a spacer. Didn't get into it too much, as I was pretty set to go the surgery route.

That's tough to hear. If you have rectal bleeding after radiation therapy, make sure you coordinate any treatments with your radiation oncologist. When I had my Proton Therapy, I was told not to have any colonoscopies, or other "fishing around" in there by anybody without them coordinating with my treating doctor at UFHPTI. There's a reason for this. Most proctitis symptoms will take care of themselves without intervention by a gastroenterologist. The rectal wall likely is healing itself and very susceptible to damage by unnecessary probing around. Be cautious. Be careful. Before my treatment, UFHPTI made sure I had undergone a recent colonoscopy so a repeat wouldn't be necessary for 4 to 5 years. Again, it is so there's no poking around in there that could cause more damage where the proctitis will likely heal on its own if left alone. So please coordinate with your treating radiation oncologist. In my case it is Dr. Bill Mendenhall. He happens to have a background in Gastrointestinal Radiation Oncology, so I'm going with his recommendations. And no, I didn't have a gel spacer inserted because the doctor called it off during the procedure. I had to go with the endorectal balloon. After 10 months, I'm seeing a little bleeding after some bowel movements and suspect a mild proctitis, as does Dr. Mendenhall. I'm keeping him up to date. No interventions are scheduled at this time.

One Year and two months after SBRT.
I had a colonoscopy yesterday. No signs of any radiation burning anywhere. My gastro said it was because of the SpaceOar's protection

I suspect that since I didn't have the gel spacer (failed procedure), I have some radiation proctitis. But it is likely to heal on its own, according to my doctor. We'll see, right? No pain, just occasional bleeding. For instance, after sleeping all night, I got up the other morning and passed about two tablespoons of blood, clotting, etc. But nothing since then. I will fast for a couple of days to give everything down there a rest. I'm thinking I'll stop riding my ebike for awhile too and will figure out some other exercise that doesn't apply any pressure to that region of concern.