Evaluation of dosimetric and spatial accuracy of a virtual cone technique for radiosurgery using linac-integrated CBCT-based polymer gel dosimetry

Abstract

Purpose: This study evaluates the dosimetric and geometric precision of a virtual cone technique using CBCT-based polymer gel dosimetry, enabling radiation delivery, and imaging readout within an identical spatial coordinate system. Methods: We created a C# script for a virtual cone technique that generates a treatment plan with 10 gantry arcs at 0°, 36°, 72°, 288°, and 324° couch angles, with 2 arcs per couch angle using 45° and 135° collimator angles. Two verification plans using Eclipse v15.6 (AcurosXB) were created with 20 Gy at the maximum dose for: (1) a cylindrical gel, with an additional calibration region; (2) a 3D printed anthropomorphic skull phantom with a gel insert. The 50% isodose (10 Gy) width through the central axis of the axial and sagittal planes (SPs) were measured for the gel experiment. The distance between the centers-of-masses of the 10 Gy isodose region of the plan and the gel (skull phantom) were calculated for an end-to-end spatial accuracy test. Results: The maximum point dose measured with gel was within 1% of the plan, though the gel measured 50% isodose widths of 5.56 (Formula presented.) 0.02 mm, 5.65 (Formula presented.) 0.04 mm, 4.23 (Formula presented.) 0.01 mm for axial (anterior–posterior), axial (left–right), sagittal (superior–inferior) respectively, which were slightly narrower than Eclipse (1.29 mm maximum difference in the SP due to CBCT slice thickness). The center-of-mass distance was 0.66 mm for the gel experiment, and 0.94 mm for complete end-to-end testing with the anthropomorphic phantom, including CBCT setup (kV-MV isocenter uncertainty). Conclusion: The 50% isodose width of the gel measurement was 5.15 mm (mean), which was tighter than our Eclipse v15.6 beam model. The end-to-end spatial accuracy test, only achievable with gel dosimetry using CBCT readout, resulted in sub-millimeter accuracy. This study demonstrates the value of gel dosimetry in verifying the dosimetric and spatial accuracy of this high precision, stereotactic technique.