That is what is done for radiotherapy. Modern techniques such as conformal radiotherapy do exactly that, re-shaping and re-directing the beam to avoid sensitive off-target areas and maximise dose to the target where all the beams collide. In practice, there is still cobsiderable off-target exposure.

For very high precision there are variants such as stereotactic radiosurgery, where up to 200 separate beams target a small area allow extremely high dose at the target with less off target dose. However, stereotactic techniques are limited in target size, and high grade gliomas are usually too large to be practically treated this way.

he other problem with gliomas is that they infiltrate, sending out microscopic "tendrils' into the adjacent brain tissue which can be much larger than the visible tumour. This is a major provlem for treatment as the tumor can recur elsewhere in the brain.