mesh deformation

I want to create a brain surgery simulator. I have a 3d brain model in 3d max. I need to change the mesh geometry of the based model to create wholes in that model when user clicks on any part of the model like drilling the brain. That should happen in real-time and the speed of rendering is important for me. What can i do? Tell me something to start plz.

This is a non-trivial thing to code. You might take a look at Mega-Fiers in the Asset Store. They have a lot of mesh modification components, and they can be combined.