How do you contour the cauda equina for a lumbar spine SBRT case below the spinal cord? Do you add a PRV on this?

I typically would contour the cauda equina at the level of the upper lumbar spine just below the spinal cord based on T2 weighted MRI and utilize a 2mm PRV. If the entire spinal canal is instead contoured just below the cord, this may lead to underdosing of the posterior aspect of the vertebral body...