In patients with skin squamous cell carcinoma invading the skull, what dose do you limit the brain when treating definitively with radiation?  

I use 1.2 Gy/fx BID to 72 Gy to treat the gross disease invasion/perineural gross disease. That translates to a dose to the adjacent brain of EQD2 of 67.2 Gy for alpha/beta = 10 and 60.48 Gy for alpha/beta = 3.

Brain (excluding optic structures and brain stem) can handle all sorts of doses to limited volumes. I would just try to avoid dumping hotspots < 105-110% of your Rx dose into the brain and try to minimize the Rx dose, which will be dependent on anatomy and the extent of abutment/overlap of PTV w...