Purpose: There are no effective treatment strategies for children with highest-risk posterior fossa ependymoma (PFA). Chromosome 1q gains (1q+) are present in approximately 25% of newly diagnosed PFA tumors and this number doubles at recurrence. Seventy percent of children with chromosome 1q+ PFA will die because of the tumor, highlighting the urgent need to develop new therapeutic strategies for this population. Experimental Design: In this study we utilize 1q+ PFA in vitro and in vivo models to test the efficacy of combination radiation and chemotherapy in a preclinical setting. Results: 5FU enhances radiotherapy in 1q+ PFA cell lines. Specifically, 5FU increases p53 activity mediated by the extra copy of UCK2 located on chromosome 1q in 1q+ PFA. Experimental down regulation of UCK2 resulted in decreased 5FU sensitivity in 1q+ PFA cells. In in vitro studies, combination of 5FU, tretinoin (ATRA) and radiation provided the greatest reduction in cellular proliferation and greatest increase in markers of apoptosis in 1q+ PFA cell lines compared to other treatment arms. Similarly in vivo experiments demonstrated significant enhancement of survival in mice treated with combination radiation and 5FU and ATRA. Conclusions:These results are the first to identify a chromosome 1q+ specific therapy approach in 1q+ PFA. Existing phase 1 studies have already established single-agent pediatric safety and dosages of 5FU and ATRA, allowing for expedited clinical application as phase 2 trials for children with high-risk PFA.