Treatment Plan Quality Comparison between Zap-Axon Optimizer and Leksell Gamma Knife Lightning Optimizer for Large Single Brain Metastasis
Abstract
Purpose
Stereotactic radiosurgery (SRS) plan optimization requires balancing high-quality dosimetry and treatment efficiency. This study compares treatment plan quality between the recently launched Zap-Axon optimizer and the Leksell Gamma Knife (GK) Lightning optimizer for large single metastatic brain tumors treated with volume-staged SRS.
Methods
Ten (n=10) patients treated with staged SRS using Gamma Knife were selected. The first stage treatment plans were independently recomputed using Zap-Axon and GK Lightning optimizers. Isocenters (Zap-Axon) or shots (GK Lightning) were automatically placed, and no manual adjustments were made before or after the optimizers were run. Optimization time per iteration (sec) was recorded for each system. Dosimetric metrics such as prescription isodose line (Rx IDL, %), target coverage (COV, %), Paddick conformity index (PCI), gradient index (GI) and delivery time estimates (min) were compared.
Results
Mean (minimum – maximum) target volume was 5.200 cc (3.038 cc - 8.329 cc) and prescription dose was 15 Gy in a single fraction. Mean COV, Rx IDL, GI, PCI and delivery time estimates for Zap-Axon vs. GK Lightning were 99.6% vs. 99.7% (P<0.05), 57.3% vs. 52.9% (P<0.05), 2.42 vs. 2.55 (P<0.05), 0.93 vs. 0.93 (P=0.4151), and 39.6 min vs. 35.1 min with GK reference dose rate of 3.0 Gy/min (P=0.1480), respectively. Mean optimization times per iteration were 290.2 sec for Zap-Axon vs. 36.6 sec for GK Lightning (P<0.05).
Conclusion
Minimal but statistically significant differences between the two optimizers were observed for COV, mean max dose, and GI, which are unlikely to be of clinical significance. PCI and delivery time estimates were similar for both optimizers. Optimization per iteration was more efficient with Lightning due to image resolution and dose grid size differences between CT images (Zap-Axon) and MR images (GK Lightning). Future studies incorporating total planning time comparison with expert manual adjustments may further demonstrate planning efficiency.