MRI-guided definition of cerebrospinal fluid distribution around cranial and sacral nerves: implications for brain tumors and craniospinal irradiation.

The SIOPE-Brain Tumor Group recently published a guideline on craniospinal target volume delineation for highly conformal radiotherapy. In order to spare critical structures like e.g., the lens or cochlea, highly conformal techniques can underdose the cerebrospinal fluid (CSF) in the dural reflections around cranial and sacral nerves. The purpose of this study is to generate evidence for CSF extension within the dural sheaths of the cranial and sacral nerves in order to improve accuracy in target volume delineation. Ten healthy volunteers, age 21 till 41 years, underwent an MRI-scan of the skull-base and sacral plexus. To evaluate CSF extension, cT2-weighted images with fat suppression, low signal to noise ratio and little to no motion-related artifacts were used. Two observers measured the extension of CSF from the inner table of the skull for the cranial nerves, and outside the spinal canal for the sacral nerves. CSF extension (mean distance [95% CI]) was visible within the dural sheaths surrounding the majority of the cranial nerves: optic nerve (40 mm [38-42]), trigeminal nerve (16 mm [15-19]), facial-vestibulocochlear nerve (11 mm [11-12]), glossopharyngeal-vagus-accessory nerve (7 mm [7-9]) and hypoglossal nerve (8 mm [7-9]). No CSF was observed outside the spinal canal at sacral level. No significant difference between both observers was measured. This study generates evidence for significant CSF extension outside the inner table of the skull. Despite the vicinity of the lens and cochlea, we therefore recommend the inclusion of both optic nerves and internal auditory canals in the clinical target volume for craniospinal irradiation when using highly conformal delivery techniques.