Thanks for your insight.  I must have missed that day in medical school.

Not all brain tumors are the same.  John McCain has a glioblastoma multiforme, which typically begins in the frontal lobe and rapidly follows the white matter tracts, crossing the midline to the opposite frontal lobe.  It most definitely affects one's thinking ability, often aided by "radiation brain" - a dulling of the senses by radiation to the head.  He has no business making his OWN decisions, let alone ours.