washington u.s. senator john mccain undergo treatment military hospital outside washington side effect cancer therapy office say tuesday mccain 81 diagnose brain cancer summer return work soon possible statement office say senator mccain currently receive treatment walter reed medical center normal side effect ongoing cancer therapy statement say senator mccain look forward return work soon possible mccain arizona republican run unsuccessfully president 2008 find aggressive form brain tumor glioblastoma surgery july blood clot leave eye receive chemotherapy radiation treatment washington area continue work senate miss senate vote week critical vote republican tax overhaul expect senate early next week mccain absence would make difficult impossible republicans pass bill mccain re-elected sixth senate term last year senator mccain treat side effect cancer therapy