Radiologic determination of the caudal border of the spinal field in cranial spinal irradiation

Abstract

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 spinal field in Cranial Spinal Irradiation. Methods and Materials : We have reviewed the pre-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 S1 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 TI-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.