MRI determination of the vertebral termination of the dural sac tip in a South African population: clinical significance during spinal irradiation and caudal anaesthesia

Abstract

Knowledge of the approximate caudal termination of the dural sac (DS) s important, especially when placing the portal fields during lower craniospinal irradiation (CSI) and performing caudal anaesthesia. The purpose of this investigation was to determine the level of termination of the DS in relation to the spine in a group of South African patients by using magnetic resonance imaging (MRI). We retrospectively reviewed the lumbosacral MR imaging of 309 patients, in each case identifying the tip of the DS. This level was recorded in relation to the adjacent lumbosacral vertebral body i.e. upper-, middle- and lower-third and adjacent intervertebral disc. The overall mean of the DS position was at the middle third of S2. Although the caudal DS tip was at the level of S2 in the majority of patients, a notable percentage (13.9%) had a DS tip level lower than the lower third of S2, and 15.2% had levels higher than the S1-S2 intervertebral disc. This study failed to demonstrate a difference in the DS termination level, compared with the levels reported in various international studies; nor is there a statistical difference between gender, race and age. Our study shows that routine placement of the portal field at the lower border of S2 adequately treats the majority of CSI patients. However, some patients (13.9%) will be undertreated and some patients (15.2%) will be overradiated. Using spinal MRI to establish the lower border of the CSI portal field will, however, benefit patients by ensuring adequate coverage of the entire neuroaxis as well as minimising late gonadal toxicity owing to overradiation.