@jeffmarc And likewise, the long-term benefits of proton over (considerably less expensive) photon-beam radiation for overall survival and quality of life are also yet to be proven, at least for prostate cancer.

That doesn't mean that proton-beam RT itself isn't as effective as photon-beam; just that the evidence doesn't exist yet to justify the huge extra cost (in most cases).

In the U.S., a proton-beam facility is often a business venture: it's justified as long as it produces enough revenue to cover the capital and operating cost and turn a reasonable profit, so once someone builds one, they market it hard and try to keep in in constant use.

Much of the rest of the world does medicine on a traige basis, so they want hard evidence that the enormous cost of a proton-beam facility (up to 30× that of an SBRT facility) actually makes a measurable difference in the long run, vs using the money to build more SBRT facilities in under-served areas. I think they're persuaded that it does for brain cancer, but not (yet) for most other types.