Patients in this study will take a certain amount of rofecoxib by mouth either once or twice a day during treatment with radiation therapy. They will continue to take rofecoxib for 6 months after the end of radiation therapy. Different dose levels will be given to different patients based on a statistical dose escalation (increase) program run on a computer called the Continuous Reassessment Method. At least 3 patients will be treated on each dose level starting at the lowest level. All patients are required to fill out a medication diary, documenting the dose of rofecoxib they are taking and the time they take it.