Ask the SpaceOAR reps to give a presentation and they'll tell you it will not only eliminate the 15% rectal toxicity you're "actually" currently seeing, but also decrease urinary toxicity (somehow, by pushing the prostate anteriorly into the bladder), and will increase potency.
True story.
When they went into the potency "data" I just kind of laughed and told them they should market it in the same space as Viagra, but that I now didn't believe a single word they said.
Pro is your rectal DVH will be better.
Con is you have to implant it, and HAVE to plan on MRI. CBCTs become difficult to interpret without fiducials, so I think you need them as well. I'm assuming there are no deleterious effects of implanting a dissolving hydrogel 20 years down the road, but who the hell knows?
Best use I see for it is after a seed implant. You're in there anyway with the whole setup. Might as well just place it at that time. Maybe before SBRT, though, if you have a man willing to undergo a TRUS and needle insertion of a foreign body via the perineum prior to SBRT... why wouldn't you just put in about 24 more needles and leave behind 120 or so seeds?
FWIW, I would never begrudge someone for doing it. I just have never viewed my late rectal toxicity as an issue (knock on wood), at least not above the risk of an additional invasive procedure (anesthesia, infection, bleeding, whatever...).