Beginning EBRT next week. Do I need to follow a low-fiber diet?

What kind of EBRT? I had 20 sessions of SBRT in 2022. I just ate my normal diet, and had no digestive or gas issues diring the therapy (I did get urinary tract irritation a year later). Your mileage may vary.

I never heard of 20 sessions Of SBRT, Usually it’s IMRT when it’s that many sessions.

Was this when they were working on your spine or Did you just get a bunch of metastasis Zapped at once?

5 sessions of SBRT on my spine post surgery (total radiation 20 gy). 20 sessions of SBRT on my prostate (total radiation 60 gy).

Miralax once per day is good; also HEALTHY FIBER taken 2-3X daily(Amazon). These are both non-bloating, non gassy. Stay away from Metamucil or anything with psyllium - very gas inducing!!
Also, on those days you simply can’t have a BM, don’t hesitate to use a glycerin suppository or a saline Fleet enema.
A clean rectum with no gas is ESSENTIAL to avoiding radiation proctitis. Ask your RO if he is doing a Cone beam Xray before each treatment; that tells the techs if it is safe to proceed with treatment; this is usually called IGRT - Intensity Guided Radiation Treatment.

My experience with diet for salvage radiation treatment was more mentally challenging than physical.
Everyone is different in bowel management.
I reduced my vegetable intake; eliminating as much and as many as possible of the identified "gas producers" such as beans and brocolli, and took a GasX at night. My normal diet included a large amount of vegetables, so I cut back significantly.
And "held my breath" on the table while my bladder and bowel prep was checked by the machine.
My best advice is to follow the diet as closely as possible or tolerable, and adjust as necessary.
I could not find a bladder or a bowel gauge at the auto parts store, so my management was guesswork and hope.
I am sure that you will discover what works best for you. Best wishes.

Heavyphil, as usual you are the man! Thanks.

I completed five rads of SBRT in May 2024. I was advised to eat a low-fiber diet too. It was a change for me, as I normally eat a lot fruits and veggies. So, I was given a "menu" of low fiber fruits and vegetables, and was told by the RO's nurse to "eat like you're back in college" for a few weeks. It took a few days for my body to adjust, but I didn't have any bowel issues during prep/simulation or during the "big show". As a practical matter, since I do most of my bowel movements in the morning before noon, I was also able to schedule my sessions for late morning or early afternoon.

No diet changes here. I take a half dose of MiraLAX in the evening and morning. In the morning, I mix 2 teaspoons of Chia seeds, which are a great fiber source, in with my Almond milk and let sit for 15 minutes. Then I add that into my small blender with a serving of protein mix and raspberries. Two hours later the magic happens before my Rad session. With each of us being different you might need to find your combo that works. No one wants to get thrown off the table into pee or bowel timeout. I heard only two men in the world can have a bowel movement without peeing lol.

Your numbers are the exact opposite of normal. They would give you five sessions to your prostate and then 20 sessions to other parts of your body. It doesn’t take 20 sessions of SBRT to totally radiate a prostate. It only takes five.

That’s what puzzles me my brother had five sessions SBRT to his prostate and it’s effectively gone. I know many other people that have had those five sessions. I can’t imagine how 20 sessions could be on a prostate.

What was your brother's total dose of radiation over those 5 sessions?