DETERMINATION OF THE INFERIOR BORDER OF THE THECAL SAC USING MAGNETIC-RESONANCE-IMAGING - IMPLICATIONS ON RADIATION-THERAPY TREATMENT PLANNING

Purpose: To determine whether the traditional teaching of placing the caudal border of the spinal field at the S2-S3 interspace in children receiving craniospinal irradiation (CSI) is appropriate. Methods and M aterials: Twenty-three children had magnetic resonance imaging (MRI) o f the spine with gadolinium prior to craniospinal irradiation at one i nstitution. Thecal sac termination using MRI was determined by drawing a perpendicular line from the point of convergence of dural margins t o the corresponding vertebral body. Results: Location of thecal sac te rmination varied from mid-S1 to low S3 vertebral body, with the most f requent site at the upper S2 vertebral level. Only 2 of 23 (8.7%) chil dren had thecal sac terminations below the S2-S3 interspace. For the n ine patients with neuraxis disease, none had thecal sac terminations b elow the S2-S3 interspace, In seven of the nine patients who had neura xis seeding at initial presentation, MRI of the spine after CSI was pe rformed and showed that thecal sac termination was lower after radiati on therapy in two children, higher in one, and the same in four. Concl usions: In 2 of 23 children (8.7%), placement of the inferior border a t the bottom of the S2 vertebral body would have missed the entire the cal sac. Treatment to the entire neuraxis with adequate coverage of di stal spinal theca can be achieved by using MRI, Individualized spinal fields using the MRI may help minimize radiation scatter to the gonads while adequately covering the target volume. (C) 1998 Elsevier Scienc e Inc.