SU-E-T-487: In VMAT of Spine Stereotactic Radiosurgery a 1 Mm Grid Size Increases Dose Gradient and Lowers Cord Dose Significantly Relative to a 2.5 Mm Grid Size




Sharp dose gradients between the target and the spinal cord are critical to achieve dose constraints in spine stereotactic radiosurgery (SRS). In this study, the volume averaging effect of grid size (GS) on the dosimetric accuracy of volumetric modulated arc therapy (VMAT) spine SRS plans was investigated.


The Eclipse v11.0 Anisotropic Analytical Algorithm (AAA) algorithm was used for dose calculation. Plan qualities of 10 treatment plans were evaluated with GS of 2.5mm (AAA's default value) and 1mm. All plans were prescribed 18Gy to the 90% isodose line. Parameters used for comparison included the distance between 18Gy and 10Gy isodose levels in the axial plane, maximum cord dose (Dmax, defined as dose to 0.035cc), dose to 10% of the cord (D10%) and 0.35cc of the cord (D0.35cc), film gamma pass rate (3%, 1mm), film line profile through the cord, and calculation time. Paired t-test was used to investigate the statistical significance.


The 18Gy−10Gy distance was shorter for all plans with 1mm compared to 2.5mm GS (0.32±0.07mm vs. 0.38±0.07mm, p<0.001). In addition, 1mm GS plans showed lower cord Dmax (11.11±1.74Gy vs. 11.96±1.6Gy, p<0.001), D10% (8.26±1.08Gy vs. 9.11±1.18Gy, p<0.001) and D0.35cc (8.55±1.28Gy vs. 9.43±1.33Gy, p<0.001). Film analysis demonstrated better agreement in the high dose gradient near the cord. Calculation times for 1mm GS plans increased significantly (14:00 vs. 2:30, p<0.001).


Due to more accurate dose calculations at the sharp dose gradient near the cord, we recommend the use of 1mm grid size for final dose calculation for VMAT SRS spine plans. This not only leads to more accurate dose gradient calculation near the cord, and lower spinal cord dose, but also, affords more room to balance between the needs of PTV coverage and cord sparing.