Author: John Bennet, Indrin J. Chetty, Tai H. Dou 👨🔬
Affiliation: Department of Radiation Oncology,Cedars Sinai Medical Center, Department of Radiation Oncology, Cedars Sinai Medical Center, Department of Radiation Oncology,Cedars-Sinai Medical Center 🌍
Purpose:
To investigate the effect of complexity of stereotactic radiosurgery (SRS) volumetric modulated arc therapy plans on gamma pass rates (GPR) using a novel high-resolution ion chamber array system.
Methods:
A set of five clinical SRS/SRT patient cases (termed Clinical) was re-optimized using larger modulation factor by experienced SRS planners to generate another set of plans of higher modulation factors, here termed HMF. Each of the five plans involves abutting OARs, such as brainstem and cochlea. The modulation factor is defined here as the ratio of the total plan monitor units to the prescription dose. The re-optimization was performed such that all clinical planning metrics are satisfied in the HMF plans. The two sets of plans were delivered on the Varian TrueBeam linac and measured using PTW Octavius 1600 SRS detectors, consisting of liquid-filled ion chambers at 2.5mm by 2.5mm resolution in the high-resolution inner area. The comparison between the measured and planned dose distributions was analyzed using VeriSoft and GPR evaluated using 2%/2mm criteria and 10% dose threshold. The resulting comparison in GPR was made by subtracting the HMF GPRs from those of Clinical plans.
Results:
The range of modulation factors in the Clinical plans ranges from 2.2 to 2.8. Relative to the Clinical plans, the increase in modulation of the HMF plans ranges from 1.41 to 1.86. The range of GPR in the Clinical plans was between 96.5% and 99.9%, whereas that in the HMF plans between 95.3% and 98.5%. Compared to the Clinical plans, the GPR in the HMF plans was somewhat lower, with the differences in GPR ranging from -4.6% to 1.2%.
Conclusion:
Despite the larger modulation factors, the tested measurement system achieves GPR at 95% and higher. Our study demonstrates that this high-resolution detector system is a valuable quality assurance tool for complex SRS plans.