The Dancing Cord: Inherent Spinal Cord Motion and Its Effect on Cord Dose in Spine Stereotactic Body Radiation Therapy

BACKGROUND: Spinal cord dose limits are critically important for the safe practice of spine stereotactic body radiotherapy (SBRT). However, the effect of inherent spinal cord motion on cord dose in SBRT is unknown. OBJECTIVE: To assess the effects of cord motion on spinal cord dose in SBRT. METHODS: Dynamic balanced fast field echo (BFFE) magnetic resonance imaging (MRI) was obtained in 21 spine metastasis patients treated with SBRT. Planning computed tomography (CT), conventional static T2-weighted MRI, BFFE MRI, and dose planning data were coregistered. Spinal cord from the dynamic BFFE images (cord(dyn)) was compared with the T2-weighted MRI (cord(stat)) to analyze motion of cord(dyn) beyond the cord(stat) (Dice coefficient, Jaccard index), and beyond cord(stat) with added planning organ at risk volume (PRV) margins. Cord dose was compared between cord(stat), and cord(dyn) (Wilcoxon signed-rank test). RESULTS: Dice coefficient (0.70-0.95, median 0.87) and Jaccard index (0.54-0.90, median 0.77) demonstrated motion of cord(dyn) beyond cord(stat). In 62% of the patients (13/21), the dose to cord(dyn) exceeded that of cord(stat) by 0.6% to 13.8% (median 4.3%). The cord(dyn) spatially excursed outside the 1-mm PRV margin of cord(stat) in 9 patients (43%); among these dose to cord(dyn) exceeded dose to cord(stat) + 1-mm PRV margin in 78% of the patients (7/9). Cord(dyn) did not excurse outside the 1.5-mm or 2-mm PRV cord cord(stat) margin. CONCLUSION: Spinal cord motion may contribute to increases in radiation dose to the cord from SBRT for spine metastasis. A PRV margin of at least 1.5 to 2 mm surrounding the cord should be strongly considered to account for inherent spinal cord motion.