It is of interest to determine whether COX-2 inhibitors given with radiation therapy can prolong the progression-free survival in brain stem glioma. Diffuse pontine brainstem gliomas are more common in children, but are also seen in adults. However, the use of commercially available COX-2 inhibitors has not been evaluated in the pediatric population and the proper dosing in pediatrics is unknown. Therefore a Phase I study will need to be conducted as a first step. Rofecoxib is an FDA approved COX-2 inhibitor for use in adults. This phase I study is designed to determine the maximum tolerated dose of Rofecoxib given concurrently with standard radiation therapy for diffuse pontine brainstem glioma.