No horsin’ around.

Top Line: How should you contour the thecal sac for lumbosacral SBRT?

The Study: Last year we saw a helpful expert consensus recommendation for contouring sacral spine mets for SBRT. An important component of that recommendation was to contour the thecal sac, cauda equina, and intra-canal nerve roots. While that may seem simple, do you draw the actual thecal sac? The bony canal? Do you need to add a PRV? And what do you do about those nerve roots as they exit the bony spinal canal into the sacral plexus? Be over-conservative and you could unnecessarily under-dose disease. Be over-aggressive and you could see a major adverse event. This follow-up study confirms that TS contouring is no easy process. Among 8 international experts in spine SBRT, there was considerable variation in contouring the “thecal sac” organ-at-risk ranging from the TS itself to the bony canal +/- individual nerve roots.  This variation led to a consensus recommendation for TS contouring. First, the TS should be defined as an OAR and spared during optimization all the way down to the caudal aspect of S5. The TS, and not intrathecal nerves, should be contoured as an OAR. You can often see individual nerve roots leave the TS and enter the neural foramina. While some experts included these roots until they left the canal space, the final consensus contour excluded these from the TS OAR. Below the termination of the anatomic TS, the bony sacral canal should be continued as the TS OAR in order to protect intracanal nerve roots. Last, the TS OAR does not require a PRV expansion (like you should do for the spinal cord) when planning and setting maximum dose constraints.

TBL: Expert consensus recommendations including images and a step-by-step guide can improve the quality and consistency of your lumbosacral spine SBRT plans. | Dunne, Int J Radiat Oncol Biol Phys 2021