Thecal Sac Contouring as a Surrogate for the Cauda Equina and Intra-Canal Spinal Nerve Roots for Spine Stereotactic Body Radiotherapy (SBRT): Contour Variability and Recommendations for Safe Practice. International journal of radiation oncology, biology, physics Dunne, E. M., Lo, S., Liu, M. C., Bergman, A., Kosztyla, R., Chang, E. L., Chang, U., Chao, S. T., Dea, N., Faruqi, M. S., Ghia, A. J., Redmond, K. J., Soltys, S. G., Sahgal, A. 2021

Abstract

PURPOSE: To present inter observer variability (IOV) in thecal sac (TS) delineation based on contours generated by eight experienced spine stereotactic body radiotherapy (SBRT) radiation oncologists, and propose contouring recommendations to standardize practice.METHODS AND MATERIALS: In the setting of a larger contouring study that reported target volume delineation guidelines specific to sacral metastases, eight academically based radiation oncologists (RO) with dedicated spine SBRT programs independently contoured the TS as a surrogate for the cauda equina and intra-canal spinal nerve roots. Uniform treatment planning simulation CT datasets fused with T1, T2 and T1 post gadolinium magnetic resonance imaging (MRI) for each case were distributed to each RO. All contours were analysed and agreement was calculated using both Dice Similarity Coefficient (DSC) and simultaneous truth and performance level estimation (STAPLE) with kappa statistics.RESULTS: A fair level of STAPLE agreement was observed between practitioners according to a mean kappa agreement of 0.38 (range, 0.21 - 0.55) and the mean DSC (± standard deviation; with range) was 0.43 (0.36 ± 0.1 - 0.53 ± 0.1). Recommendations for a reference TS contour, accounting for the variations in practice observed in this study, include: contouring the TS to encompass all the intra-thecal spinal nerve roots and, caudal to the termination of the TS, the bony canal can be contoured as a surrogate for the extra thecal nerves roots that run within it.CONCLUSION: This study shows that even amongst high volume practitioners, there is a lack of uniformity when contouring the TS. Further modifications may be required once dosimetric data on nerve tolerance to ablative doses, and pattern of failure analyses of clinical datasets utilizing these recommendations, become available. The contouring recommendations were designed as a guide to enable consistent and safe contouring across general practice.

View details for DOI 10.1016/j.ijrobp.2021.08.023

View details for PubMedID 34454046