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 cont...