Vircet, I did not have SBRT but everyone I know or met in the hospital who did have a 5 session protocol, had a gel spacer.
Fiducials only help the machine track the prostate - they do not block any radiation from getting to the rectum.
Beams pass right thru the prostate and hit everything before passing thru the body - the rectum and bladder being the most susceptible to damage.
I had 25 salvage treatments and could not have a spacer and even though the radiation per session is less than that of SBRT, the damage is still cumulative. So I could still have proctitis down the road, even though you are experiencing it now. No one is unscathed! Best of luck to you,
Phil

I haven't had any negative surprises as of this date. (fingers crossed) Keep in mind, my radiation was salvage in nature, likely less than primary treatment type radiation.