A factor not mentioned so far is the role of the physicists in radiation therapy. At least when I had my IGRT done at Vanderbilt, they had three physicists on staff to design the actual radiation plans. They work with the x-ray machine's software, and the MRI and CT scans of a patient's physiology, to develop the treatment field. I got a copy of my actual treatment plan, a color map of the dosage plan
We never meet the physicist, don't know their name, know nothing of their background other than it was good enough for the cancer center to have hired them! The RO will decide the photon energy level, dose schedule, what margins there should be, what areas should be treated, which are all quite important considerations. But the unsung hero in this scenario is the physicist who programs the x-ray machine to actually, accurately, precisely, deliver the intended level of radiation while minimizing injury to surrounding tissue.
I suppose that's a recommendation to have high-dose radiation therapy done at a center with significant experience and history. You'd look at facility success rates somehow, and I'm not sure where that could be found. It wouldn't be for a specific RO necessarily, but the overall success of the facility.