There is absolutely no doctor anywhere who could certify that a person afflicted with brain cancer retains his/her full mental capacity. A sane, responsible public official with such a diagnosis  would recuse himself from voting until such time that doctors can unequivocally state via definitive testing that his brain is not any longer being ravaged by  tumorous growth. A president nailed with that dread disease would either step down or be forced to via the 25th amendment along with a responsible Vice president.
McCain was off the wall before he was diagnosed. Now, with tumorous  tissue wrapping around his kooky brain there he is rendering far reaching highly consequential votes and uttering nonsensical statements and criticisms that the lowlife media ballyhoos 24/7. Somehow, someway he has to be made to step aside until such time that he can gain medical assertion that 1. The advance of the cancer's tentacles has been arrested and 2, That it has not caused  cognitive damage.