Assessing the Need for High-Resolution Array Measurements In Cyberknife Perpendicular Composite QA
Abstract
Purpose
CyberKnife stereotactic treatments are delivered using a step-and-shoot technique, with patient-specific QA verification plans for diode array measurements typically generated and evaluated in a perpendicular composite geometry. This study investigates whether the increased spatial resolution of SRS MapCHECK is required to identify delivery-related dosimetric discrepancies compared to MapCHECK® 1 for CyberKnife SRS and SBRT plans.
Methods
Twelve CyberKnife SRS and SBRT treatment plans, including lung, brain, prostate, and spine cases, were selected for this study. All plans had previously undergone patient-specific QA using MapCHECK® 1, with gamma analysis performed using 3%/2 mm criteria and a 10% dose threshold. The same patient plans were used to generate verification plans for SRS MapCHECK measurements. SRS MapCHECK was calibrated according to institutional patient-specific QA protocols, and measurements were performed under identical delivery conditions
Results
For all treatment sites, MapCHECK® 1 demonstrated agreement above 95%, with an average gamma passing rate of 98.4% ± 1.6% using the 3%/2 mm criteria. In comparison, SRS MapCHECK achieved gamma passing rates above 90%, with an average of 94.3% ± 2.7% under the same 3%/2 mm criteria. The higher diode concentration of the SRS MapCHECK enables detection of greater dose discrepancies; however, the measured values remain above the recommended action level.
Conclusion
For CyberKnife perpendicular composite patient-specific QA, both MapCHECK® 1 and SRS MapCHECK met institutional acceptance criteria consistent with TG-218. While the higher detector density of SRS MapCHECK can reveal more localized dose discrepancies, it did not identify clinically significant delivery issues beyond those detected with MapCHECK® 1 in routine SRS/SBRT verification. Given its greater sensitivity to calibration and output stability, SRS MapCHECK may be most beneficial when higher spatial sampling is expected to influence clinical decisions.