Determination of the thecal sac ending using magnetic resonance imaging: clinical applications in craniospinal irradiation.

ABSTRACT

PURPOSE: To determine level of the thecal sac ending in unaffected individuals of young age-group and in patients with cerebrospinal fluid (CSF) seeding tumor using Magnetic Resonance Imaging (MRI). MATERIAL AND METHOD: MRI of the lumbosacral spine of 253 cases (age range 1-40 years) had been reviewed. Of these, 20 cases were known having CSF seeding tumors. Two hundred and thirty-three cases of unaffected individuals were used as control group. Sagittal MR images were used to determine the level of thecal sac ending by referencing on the corresponding vertebral body and intervertebral disc. RESULTS: The range of the thecal sac ending level in unaffected population varied from the lower one-third of L5 to the middle one-third of S3. About 97% of cases, the distal end of thecal sac terminated at the S2-3 intervertebral disc space or higher. In patients with CSF seeding tumor, the range of thecal sac ending was from the middle one-third of L5 to the S2-3 intervertebral disc space. There was no statistically significant difference of thecal ending level between unaffected group and patients with CSF seeding tumors (p=0.19). CONCLUSION: Placing inferior border of radiation field at the middle one-third of S3 vertebra would help to cover the entire subarachnoid space in all cases. Without MR machine, this study may be used as a general guideline for placing the inferior border for spinal irradiation.