Purpose: The ZAP-X is a novel first-of-its-kind self-shielded radiosurgery system dedicated to cranial/head and neck lesions. CyberKnife was designed for both intracranial and extracranial purposes with thousands of treatments delivered to date. The objective of this study is to compare each platform’s dosimetric characteristics when treating brain metastases.
Methods: Eleven patients with 28 brain metastases (volume range 0.06 – 4.89 cm³) previously treated using CyberKnife were selected to generate comparative ZAP-X plans using its dedicated treatment planning system. For ZAP-X planning, sphere packing was used to place isocenter positions, and beam weights were optimized using linear and quadratic programing. CyberKnife plans were optimized using an improved sequential algorithm (VOLO). For both systems, ray-tracing algorithm was used to generate beam sets and calculate final doses with high resolution. For all plans, the prescription isodose percentage was chosen to optimize target-volume coverage between 97-100%. TG101 dose constraints were utilized for all organs at risk. Conformity, homogeneity and gradient indices (CI, HI and GI) were calculated to evaluate plan quality.
Results: Comparing identical prescriptions (15-30 Gy), the median prescribed isodose lines achieved using ZAP-X were lower than those achieved using CyberKnife, median: 75% (range: 55%-88%) vs 83% (range: 80%-86%), respectively. The median CI (1.44 vs. 1.38, p=0.26) and target-volume coverage (99.79% vs. 99.81%, p=0.29) did not differ significantly between the ZAP-X and CyberKnife plans. Significant differences in the HI and GI were noted between the plans. CyberKnife produced plans with a higher median HI compared to ZAP-X (1.32 vs. 1.19, p<0.0001) while ZAP-X produced plans with a lower median GI compared to CyberKnife (3.20 vs. 5.83, p<0.0001).
Conclusion: ZAP-X and CyberKnife produced dosimetrically comparable plans that were nearly identical in target coverage and conformity. CyberKnife produced more homogenous plans compared to ZAP-X. ZAP-X produced plans with sharper peripheral dose falloff compared to CyberKnife.