Determination of the inferior border of the thecal sac using magnetic resonance imaging: implications on radiation therapy treatment planning.
OBJECTIVE
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
Twenty-three children had magnetic resonance imaging (MRI) of the spine with gadolinium prior to craniospinal irradiation at one institution. Thecal sac termination using MRI was determined by drawing a perpendicular line from the point of convergence of dural margins to the corresponding vertebral body.


RESULTS
Location of thecal sac termination varied from mid-S1 to low S3 vertebral body, with the most frequent site at the upper S2 vertebral level. Only 2 of 23 (8.7%) children had thecal sac terminations below the S2-S3 interspace. For the nine patients with neuraxis disease, none had thecal sac terminations below the S2-S3 interspace. In seven of the nine patients who had neuraxis seeding at initial presentation, MRI of the spine after CSI was performed and showed that thecal sac termination was lower after radiation therapy in two children, higher in one, and the same in four.


CONCLUSIONS
In 2 of 23 children (8.7%), placement of the inferior border at the bottom of the S2 vertebral body would have missed the entire thecal sac. Treatment to the entire neuraxis with adequate coverage of distal 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.
Scharf
C B
CB
Department of Radiotherapy, Loyola University of Chicago, Maywood, IL 60153, USA.
Paulino
A C
AC
Goldberg
K N
KN
D000293:Adolescent; D002528:Cerebellar Neoplasms / Q000532:radiotherapy*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008527:Medulloblastoma / Q000532:radiotherapy*; D012447:Sacrum / Q000033:anatomy & histology; D013131:Spine / Q000033:anatomy & histology*