Purpose: A dedicated spine stereotactic radiosurgery optimization algorithm was commissioned for 6FFF Versa HD deliveries. Plan comparisons with the existing clinical optimization algorithm along with the dosimetric validation of the deliveries were investigated.
Methods: Nine stereotactic spine patients, previously treated with Pinnacle generated plans, were re-optimized using Brainlab’s SRS spine planning Element. Three additional Elements plans were created for each patient: 1) initial optimization using a pencil beam calculation algorithm (PBC), 2) Monte Carlo (MC) re-calculation of the PBC plan, and 3) re-optimization of the PBC plan using Monte Carlo. Patient dose from PBC plans was compared to MC recalculated to evaluate dosimetric differences between Elements calculation models. MC re-optimized plans were compared to initial Pinnacle plans to evaluate optimization algorithms. Plans were compared using PTV percentage receiving prescription dose (PTV-V100%) and max spinal canal dose (Canal-DMAX). MC re-optimized plans were delivered to a microdiamond chamber and SRSMapcheck in the StereoPHAN phantom to verify deliverability.
Results: Recalculating the Elements PBC plans with Elements MC reduced the PTV-V100 by -2.5%+/-3.28%, while increasing the Canal-DMAX by 1.14Gy+/-0.50Gy. Subsequent MC re-optimization led to similar Canal-DMAX doses as the PBC calculations, for the same target coverage [Wilcoxon Rank Sum alpha<0.05]. Elements MC re-optimized plans reduced the Canal-DMAX by 3.59Gy+/-2.13Gy, while maintain the same PTV coverage compared with Pinnacle. On average, microdiamond measured point doses were within –1.01%+/-2.18% and 0.311%+/-1.20% for targets and OARs, respectively. Average per-plan pass rates using a 2%/2mm/10% threshold relative gamma analysis were 99.1%+/-0.89%.
Conclusion: Initial optimizations using PBC provided a fast method for exploring realizable optimization objectives that could be achieved with subsequent MC optimization. Brainlab’s site-specific spine SRS optimizer was able to produce deliverable plans with lower spinal canal dose [Paired T-Test P<0.001] and similar target coverage to Pinnacle.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a Clinical Cooperation Agreement with BrainLab AG, Munich, Germany.