Is there a volume limit above which you might not offer standard chemoradiation for a patient with a GBM?  

I think this requires more information to answer. I presume the question is if there is a GBM that is of a certain size for which I may not offer standard chemoRT. There may be situations for which I may not offer standard 60 Gy in 30 fx IMRT and consider hypofx RT including older patients or patien...