It's the treatment used in the PACE-B trial. Two of my support groups are in the catchment area of one of the trial centres, and they have been offering this as a standard of care for 3 years since the PACE-B trial results became clear.
The eligibility is very similar as for seed brachytheray, so they have been offering it to everyone who was offered seed brachytherapy. During this period, everyone in those support groups eligible for seed brachytherapy has chosen the 5 fraction SABR instead and has been pleased with the results, with the first one now 3 years out from treatment. (I don't know anyone who was on the PACE-B trial, but they were all 5 years after treatment 3 years ago, so treated 8+ years ago on Cyberknife, which was the only way to deliver SABR back then.)