Cull voronoi cells based on distance?

This is my first post, so hello everyone!

I've been playing with grasshopper for a while, but I still can't understand some basic stuff. So sorry if my questions are too simple, but I can't find any info about it.

I have created a 3d voronoi inside a box and now I want to get rid of all cells on the corners. My idea is to set a threshold (let say 10), and to delete those cells which lies closer than threshold (10 units) from corner points. Of course I can cull those cells when I find the indexes manually, but it's not the point. I just want to know how to manage such operations. 

oh and there is one more question - how do I measure the distances? The best way would be to measure the distance between corner points and cells centers, but I don't know how to find these centers. 

I will appreciate any help