I had SpaceOAR injected prior to my 28 sessions of proton radiation (April-May 2021).
SpaceOAR increases the perirectal space - by just over 1cm (about 1/3”). It’s easy to deal with and worthwhile moving the rectum out of the radiation field. (I’ve heard that it reduces radiation to the rectum by ~70%.)
Studies show that SpaceOAR not only reduces rectal radiation dose, but also protects from late GI and GU toxicities, with urinary, bowel, and sexual quality-of-life improvement: https://pubmed.ncbi.nlm.nih.gov/34029607/
I used the newer SpaceOAR Vue product, which is a modification of the standard SpaceOAR Hydrogel, containing approximately 1% iodine bound to the hydrogel to be more visible under CT scan. (We did not use fiducial markers.)
I had no adverse side-effects from the SpaceOAR injection. Yes, there was some bleeding and pain at the injection site (which is to be expected). I’d recommend that you ask them to be generous with the lidocaine (or whatever it is called that they use to numb the areas), both where they penetrate the skin and all the way ahead of where they’re injecting the gel.