I routinely spare uninvolved hippocampus/hippocampii in gliomas (including glioblastoma) and consider use of memantine if patient is complaining of neurocog issues. Whether it makes sense to give memantine prophylactically for glioma might as well be another clinical trial.
There was a group using whole brain MR Spectroscopy to dose paint to glioblastoma. If they're to be believed, it seems like a high density of tumors cells extend far from some tumors and aren't seen by T1+contrast and/or T2 changes.