A Dose-Painting Technique for Rotating Gamma SRS/SBRT Systems
Abstract
Purpose
Conventional sphere-packing techniques for Gamma Knife treatment plan optimization often produce less conformal dose distributions for non-spherical targets and are inefficient for dose delivery. This work investigates a dose-painting technique to improve dose conformity and delivery efficiency for rotating Gamma SRS/SBRT systems.
Methods
This technique is based on the CybeRay system (OUR United RT Group, Xian, China), which consists of a ring gantry and a focusing treatment head with 16 Co-60 sources of 6.22mm diameter. Each source has 7 collimators of 6, 9, 12, 16, 20, 25 and 35mm diameter. The treatment head can swing 35° superiorly, allowing a total of 43° non-coplanar beam incident. The kV CBCT system provides real-time target/fiducial tracking and the EPID system verifies the delivered dose for auto-quality assurance. The dose-painting technique scans the focused Gamma beams via an optimized isocenter trajectory inside the target volume combining gantry rotation (speed: 1.25-3.5º/s) and couch movement (speed: 0.03-0.47mm/s). Ten previously treated brain, lung and liver patients were re-planned with the same clinical goals and evaluated based on target coverage (%PTV receiving prescription dose), CI (prescription isodose volume/PTV) and beam-on time.
Results
Ensuring a good target coverage (97±1%) for all treatment plans, the dose-painting technique provided better dose conformity for all patients (mean CI=1.12) than the manual and auto sphere-packing techniques (mean CI=1.36 and 1.21, respectively). The beam-on time was also significantly reduced with the dose-painting technique (mean T=14.8min) compared to manual and auto sphere-packing techniques (mean T=21.3min and 17.3min, respectively). The dose-painting technique also ensured hot spots to be inside the CTV for large and irregularly shaped targets.
Conclusion
The new dose-painting technique can deliver high-quality treatments with superior target coverage, high dose conformity and significantly reduced treatment time for rotational Gamma SRS/SBRT systems compared to conventional sphere-packing techniques commonly used in Gamma Knife SRS/SRT planning.