Stereotactic Ablative Radiosurgery (SABR) of Dorsal Sacral Lateral Nerve Branches for Treatment of Sacroiliac Pain – a Dosimetric Feasibility Study Exploring the Role of Noninvasive SABR in Place of Bipolar Radiofrequency Ablation

Abstract

Sacroiliac (SI) joint dysfunction can be the underlying cause of low back pain in up to 15 to 25% cases. Bipolar RFA lesioning of the lateral branches of the S1 to S3 dorsal rami, which innervate the SI joint, can provide clinical relief of pain in patients. Due to the irregular paths of the nerves travelling to the SI joint and the many small nerve branches, it can be technically difficult to ablate the entire area at risk with RFA, which may make relief less effective. Ablative therapies such as SABR can be used for nerve ablation in settings such as trigeminal neuralgia. We hypothesize that LINAC-based SABR may be a noninvasive option for delivering ablative radiation dose to the branching dorsal lateral nerves while maintaining low doses to the surrounding organs at risk (OARs). The ultimate goal being equivalent or improved pain relief when compared to RFA. CT images of the lumbosacral spine in 1.25mm slices were obtained from 5 anonymized patients with standard immobilization who were treated at our facility for issues unrelated to back pain. An interventional radiologist who regularly performs injections and RFA contoured the dorsal nerves of the SI joint for the targeted strips of the dorsal sacral surface bilaterally. PTV volumes included a 1mm expansion. Four VMAT beams targeting the PTV were used to optimize the dose distribution and spare the OARs. The following table shows the relevant treatment volumes, PTV coverage, and OAR doses. We avoided excessive doses to the surrounding OAR structures in the plans for all 5 patients bilaterally. A thin 1mm strip of bone was included in the PTV. Excessive bone doses and volumes were minimized in an attempt to lower risks for osteoradionecrosis. Coverage was acceptable with an average of 49Gy covering the 95% IDL. This study demonstrates the feasibility of delivering ablative doses to the lateral branches of the dorsal sacral nerves that innervate the SI joint. Due to its posterior location all OARs received acceptable doses, other than the portion of bone immediately deep to the treated strip of tissue. By minimizing the treated bone volume while covering the pathways of the traveling nerves, we hope to minimize risk of osteoradionecrosis. This unique approach requires further study but has the potential to replace invasive interventions and allow for decrease in opiate requirement and consumption.Abstract 3601; Table 1Structure and parameterMean ± SDPTV volume (cm3)4.45±1.20Spinal canal Dmax (cGy)218±172Bladder mean (cGy)34±17Bowel mean (cGy)147±60Bone mean (cGy)535±147Bone volume over 30 Gy (cc)21±15 Open table in a new tab