Inr J Radmlron Oncology Bol Phys., Vol. 26, PP. 669-613 03603016/93 $6.00 + .oO Printed in the USA All rights reserved. copyright a 1993 Pergamon Press Ltd. ?? Therapeutic Oncologic Imaging zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED RADIOLOGIC DETERMINATION OF THE CAUDAL BORDER OF THE SPINAL FIELD IN CRANIAL SPINAL IRRADIATION SUSAN F. DUNBAR, M.D.,*” PATRICK D. BARNES, M.D.+ AND NANCY J. TARBELL, M.D.* Children’s Hospital, Harvard Medical School, Boston, MA Purpose: The purpose of this paper is to determine the inferior border of the caudal sac which dictates the placement of the lower border of the suinal field in Cranial Sninal Irradiation. Methods and Materials: Wk have reviewed the pie-treatment craniospinal Magnetic Resonance Imaging studies of 24 evaluable children with seeding central nervous system tumors who were treated at our institution with Cranial Spinal Irradiation since 1988. Results: The Magnetic Resonance Imaging studies demonstrated significant variation in the terminal location of the caudal sac, ranging from S2 to S4. The most frequent termination was at S2 (12/24). In four patients (4/24), termination was at mid Sl and in eight others (8/24), it was found to be at or below S3. In addition, the presence of spinal metastases may displace the distal limit even further inferiorly. Conclusion: Rather than arbitrarily placing the inferior field edge at S2, we recommend individualizing the required margin for the spinal field which should be determined using sagittal Tl-weighted images of the lumbosacral spine. This is particularly important in patients who present with spinal metastases, since tumor may extend the dural sac termination distally. Central nervous system seeding tumors, Craniospinal irradiation, Magnetic resonance imaging, Dural sac. INTRODUCTION Primary tumor location and the potential for cerebral spi- nal fluid (CSF) spread precludes surgery as the sole in- tervention in some intracranial neoplasms. In these cases, postoperative craniospinal irradiation (CSI), with or without systemic treatment, is the standard of care. Most commonly used for medulloblastoma, CSI is also fre- quently given for anaplastic ependymomas, selected germ cell tumors, and primitive neuroectodermal tumors (PNET’s). CSI is an established treatment modality. Its use in medulloblastoma was initially reported in the 1930’s (7). Most commonly, the fields include lateral whole brain fields matched to one or two posterior spinal fields, de- pending on the size of the patient and capability of the radiotherapy system. Traditionally, the caudal border of the spinal fields has been set at the second sacral segment (S2). Presumably, although not stated in the literature, this border was determined on the basis of autopsy series which located the termination of the spinal subarachnoid space (SAS) in the adult at S2 (3, 18, 19, 25). Myelography (9-12) and more recently magnetic res- onance imaging (MRI) (2,4, 12,28-30) have been used to determine the presence and extent of spinal tumor seeding. Use of these imaging studies to routinely define the lower border of the dural sac for treatment planning in individual patients has not been reported. At this in- stitution, we recently reviewed the MRIs of patients with potentially seeding Central Nervous System (CNS) tumors to determine the caudal termination of the dural sac, and to correlate these findings with treatment technique. METHODS AND MATERIALS From January 1988 to February 1992,24 children un- derwent evaluation, staging, and treatment at The Dana Farber Cancer Institute and Children’s Hospital for CNS tumors with seeding potential according to sequential in- stitutional protocols. The patients ranged in age from 2 to 17 years. There were 12 females and 12 males. All but one patient received systemic therapy and all received external beam radiotherapy. In all patients, MRI with gadolinium enhancement was done for staging and treat- ment evaluation. The MRI studies were done using a *+ The Brain Tumor Center, Joint Center for Radiation Therapy and the Department of Radiation Oncology; and De- partment of Radiology. * Dr. Dunbar is the recipient of the Winston-Lutz fellowship in stereotactic radiotherapy. 669 Reprint requests to: Nancy J. Tarbell, M.D., Department of Radiation Oncology, Children’s Hospital, 300 Longwood Ave., Harvard Medical School, Boston, MA 02 115. Accepted for publication 18 February 1993.