What margins do you use for glioblastoma in close proximity to critical structures?

Assuming standard fractionation to 60 Gy with concurrent temozolomide, I am struggling to think of a GBM patient who has had a complication due to radiation to adjacent critical structures. Since my GBM margins are not very large (Initial: 1 cm around the T2 + cavity. Cone-down: 1 cm around the contrast-enhancing T1 abnormality + cavity), it is hard to cut down further without missing the tumor. And given the very high likelihood of GBM recurrence/progression relative to the risk of radiation toxicity such as optic chiasm damage, I have not cut back on margins further. But I would definitely track the dose to the optic chiasm, and make sure there are no hot spots. I believe that most GBM cooperative group studies allowed 60 Gy to the optic ...