The study will be performed in four phases: screening, the active dosing phase, short-term follow-up and long-term follow-up. The screening phase will be performed within 4 weeks of randomization and will determine subjects' eligibility. The active phase will begin on the first day of study drug dosing (also the first day of radiation therapy) and will continue until the last day of radiation therapy. This period typically is 7 weeks depending on the subject's radiation plan. The short-term follow-up phase will begin on the last day of radiation and continue for approximately 4-6 weeks until clinical and symptomatic signs of oral mucositis have resolved. The long-term follow-up phase extends for 12 months following the last dose of radiation and is included to assure that EC-18 does not the impact of tumor response to treatment.