Besides the usual cost issues and unproven efficacy issues, GBM (ok, high grade glioma), to me, is very ill-suited for proton therapy.
Hallmark of GBM histology is its diffuse microscopic infiltrative processes. In fact, a good argument can be made that we've been missing the target all along when we target edema + margin then enhancement + margin for a boost.
The selling point of proton therapy is that it "stops" and that there is less exit dose. (By the way, I am not convinced that we can do good enough dosimentry to know exactly where it stops, nor am I convinced whether the integral dose is lower.) But, even if we have the utmost precise control of the dose deposition, we wouldn't know what to target in the case of high grade glioma. Until we can figure out what to target, we shouldn't worry too much about sparing surrounding normal tissue.
Remember the first rule of target delineation. Don't miss the tumor!