Some childhood brain tumors can be cured with aggressive cancer therapy but children who receive cranial radiation as part of their treatment often suffer a progressive and debilitating decline in cognitive function. Our earlier work showed that radiation therapy ablates stem cells in the brain and prevents ongoing brain development that normally occurs in early childhood. Those few stem cells that remain become dysfunctional, in part due to a permanent tissue inflammation caused by radiation injury. In this report, we discovered that genetic deletion of a single immune signaling molecule in mice reduced inflammation and allowed the remaining neural stem cells to recover their normal function and resume neurogenesis. These results suggest that modifying inflammation in the brain following therapy may reduce or even prevent the cognitive decline that plagues otherwise successful treatment of childhood cancers.