A Beam Orientation Quality Assurance Framework for Gyroscopic Stereotactic Radiosurgery Systems

Gyroscopic stereotactic radiosurgery (SRS) systems, such as ZAP-X, enable precise, isocentric beam delivery with high conformality and minimal exposure to surrounding tissues. While standard quality assurance (QA) protocols verify beam centricity and overall targeting accuracy, they are not specifically designed to assess the consistency of beam entry angles, which can vary in systems employing gimbaled delivery. These angular variations, though often less consequential at the target, may have significant dosimetric implications for peripheral critical structures. To address this, we developed a practical quality assurance framework using the ArcCHECK cylindrical diode array for periodic beam orientation monitoring. The system comprises 1,386 diodes arranged cylindrically (21 cm diameter), providing a density of 221 diodes per 10×10 cm². Although the array's resolution limits direct dose distribution comparisons, its sensitivity to angular deviations leverages the steep dose fall-off typical in SRS beams. A calibration protocol was established using a 44-beam QA plan with collimators of varying sizes. Sensitivity to beam rotation was quantified as the percentage of diode failures per degree of angular deviation. The 10 mm collimator demonstrated optimal performance, with sensitivity reaching 37.7% per degree, owing to the match between its penumbra and the 1 cm diode spacing. Daily QA measurements confirmed high reproducibility and sensitivity, with a deliberately introduced 0.8° error accurately detected and quantified. This framework, validated through controlled tests, offers a reliable and efficient method to track beam orientation consistency without requiring additional hardware or major workflow changes. While the study serves as a proof of principle within a single institution, broader adoption and multicenter validation could support its standardization. Ultimately, the integration of angular QA enhances treatment fidelity in gyroscopic SRS, ensuring both mechanical precision and patient safety.