Radionecrosis is a potentially devastating complication of external beam radiotherapy (XRT). Intraventricular compartmental radioimmunotherapy (cRIT) using (131)I-3F8 or (131)I-8H9 can eradicate malignant cells in the CSF. The incidence of radionecrosis using cRIT (131)I based intraventricular radioimmunotherapy, when used alone or in combination with conventional craniospinal CSI-XRT is unknown. We retrospectively analyzed the incidence of radionecrosis in two cohorts of pediatric patients treated with both CSI-XRT and cRIT at MSKCC since 2003: patients with metastatic CNS neuroblastoma (NB) and medulloblastoma (MB). 94 patients received both CSI-XRT and cRIT, two received cRIT alone, median follow up 41.5 months (6.5-124.8 months). Mean CSI-XRT dose was 28 Gy (boost to the primary tumor site up to 54 Gy) in the MB cohort, and CSI XRT dose 18-21 Gy (boost to 30 Gy for focal parenchymal mass) in the NB cohort. For MB patients, 20 % had focal re-irradiation for a second or more subsequent relapse, mean repeat-XRT dose was 27.5 Gy; seven patients with NB had additional focal XRT. Median CSF cRIT dose was 18.6 Gy in the MB cohort and 32.1 in the NB cohort. One asymptomatic patient underwent resection of 0.6-cm hemorrhagic periventricular white-matter lesion confirmed to be necrosis and granulation tissue, 2.5 years after XRT. The risk of radionecrosis in children treated with XRT and cRIT appears minimal (~1 %). No neurologic deficits secondary to radionecrosis have been observed in long-term survivors treated with both modalities, including patients who underwent re-XRT. Administration of cRIT may safely proceed in patients treated with conventional radiotherapy without appearing to increase the risk of radionecrosis.