Extended Field Radiosurgery of Spinal Metastases with MRI-Guided Adaptive Planning: A Dosimetric Study

Abstract

Spinal metastases occur in ∼40% of cancer patients. Stereotactic body radiotherapy (SBRT) precisely delivers high doses of RT to enhance local control and reduce chronic pain. Eligibility for spine SBRT can depend on number of spine vertebrae involved. For example, RTOG 0631 was limited to two contiguous levels due to concerns about spinal cord (SC) tolerance when aligning to long targets in the mobile spine. The purpose of this study was to determine whether adaptive replanning on an MRI-Linear Accelerator could permit SBRT of longer spinal segments. We randomly selected ten patients treated in the abdomen on a 0.35T MRI-Linac with MRI including L2-T8. For each patient, their simulation MRI was used to generate six distinct plans starting at 2 spinal levels incrementing to 7 spinal levels. Coverage was 90% at 16 Gy to the entire vertebrae including processes and TG-101 constraints were used for SC and other nearby organs at risk (OAR). Each plan was then rigidly fused in 6-dimensions to a treatment MRI obtained on another day (non-adaptive) and evaluated. An adaptive plan was then generated for each non-adaptive plan. Target coverage between non-adaptive and adaptive techniques was compared using paired t tests. McNemar's tests were used to compare proportions of dose objective failures for OAR between sets. The target-coverage and number of patients meeting V10 Gy <0.35 and Dose max <14 Gy are shown in the table below with respective p-values. Statistically significant p-values (<0.05) were obtained for all criteria analyzed, especially above 4 vertebral body (VB) levels. This dosimetric analysis suggests that adaptive replanning using a hybrid MR/Linac platform has the potential to safely extend spine SRS beyond the currently accepted practice of 2-3 consecutive VB levels. Future directions include applying this innovative approach in a clinical trial.