I had this done. Brand names are SpaceOAR or Barrigel. It creates space between the rectum and prostate. In my case 1.7 cm. Considering my SBRT radiation margins are 3mm in that area my rectum should be at least somewhat protected from radiation toxicity. I had mine placed under general anesthesia along with my fiducial markers. Going for fourth SBRT in 2 hours, so far zero side effects (fingers crossed).

Stay Strong Brother