Purpose: Craniospinal irradiation (CSI) is employed in medulloblastoma and other intracranial malignancies that can seed the neuroaxis. Care must be taken to adequately cover the entire craniospinal axis, including the distal thecal sac. The inferior border of the craniospinal field has traditionally been placed at the bottom of the S2 vertebra. The purpose of this study is to review the level of thecal sac termination in children undergoing CSI using Magnetic Resonance Imaging (MRI).Methods and Materials: From 1287 to 1095, 22 children were treated with CSI at one institution. All underwent pre-treatment MRI of the spine with Gadolinium as part of their evaluation. The median age was 9 years (range, 31 months to 18 years), and there were 14 males and 8 females. The diagnosis was medulloblastoma in 14 patients, primitive neuroectodermal tumor of the cerebrum in 3, germinoma in 2, pineoblastoma in 1, leptomeningeal gliomatosis in 1 and glioblastoma multiforme in 1. All spinal MRIs were reviewed by both neuroradiologist and radiation oncologist to accurately determine the level of thecal sac termination which was obtained by drawing a horizontal line from the lower limit of the spinal theca to the corresponding adjacent vertebral body.Results: The thecal sac termination varied from mid S1 to lower S3. It was located at mid S1 in 1 patient, lower S1 in 3, S1-2 junction in 3, upper S2 in 5, mid S2 in 3, lower S2 in 3, S2-3 junction in 2, upper S3 in 1 and lower S3 in 1. Only 222 patients (9%) had thecal sac terminations below the S2-3 junction. Eight patients had spinal axis involvement and their thecal sac terminations were all above the S2-3 junction. There was no correlation between the level of termination and age, gender or histology.Conclusions: The majority of patients (91%) will have termination of the thecal sac above S3, and therefore placement of the inferior border of the spinal field at the S2-3 junction with a 1 cm caudal margin will be adequate coverage for most cases. MRI of the spine is still of paramount importance in determining spinal axis seeding. It may not be necessary in the placement of the caudal border of the craniospinal field in all patients; however, it may still have a role in young females whose ovarian function may be compromised.