TDD AM PM Placebo 2 placebo 2 placebo 500mg 1 active + 1 placebo 2 placebo 1000mg 1 active + 1 placebo 1 active + 1 placebo 2000mg 2 active 2 active Dosing will commence on the first day of radiation (one hour after the first fraction) and continue until the last day of radiation. Test drug will be administered as an oral 500 mg capsule in divided daily doses as indicated above. At the completion of 4 weeks of dosing, an independent Data Safety Monitoring Board (DSMB) will evaluate safety endpoints in blinded fashion. If no safety issues are identified, the DSMB will approve continuation of dosing until the last day of radiation. If a safety issue is noted, the DSMB may unblind the treatment assignment to ascertain if the adverse event is associated with study drug. DSMB assessment will be repeated after completion of dosing (when each subject has been dosed through the last day of radiation). If no safety issues are identified, the efficacy component of the study (Stage 2) will commence using the highest dose of study drug consistent with a positive safety outcome.