Development of an SRS Audit Program
Abstract
Purpose
Stereotactic radiosurgery (SRS) delivered with linear accelerators requires submillimetric mechanical, geometric, and dosimetric accuracy. Although postal and remote audit programs are widely available, they do not fully capture system performance under clinical conditions. This study introduces a truly integrated on-site audit framework for cranial SRS that combines advanced small-field dosimetry, submillimetric geometric verification, and volumetric PTV dose assessment in a single clinical intervention. The program is aligned with AAPM TG-142, TG-198, and IAEA TRS-483 and TRS-489 recommendations.
Methods
The audit was structured into two sequential phases: pre-audit, on-site audit. Pre-audit testing included SRS-specific QA procedures such as Winston–Lutz, Picket Fence, and output factor determination for small fields. The on-site audit was performed using the CIRS STEEV2 phantom with multiple inserts, allowing simultaneous evaluation of spherical and irregular targets. Absolute dose measurements were obtained using microdiamond chambers, while two-dimensional dose distributions were evaluated using radiochromic films. IMRT/VMAT treatment plans were generated following institutional PSQA protocols. Dose agreement within the PTV and spatial dose accuracy were assessed using stringent gamma criteria of 2%/1 mm per TG-198. Results were summarized using dose distribution diagrams and quantitative comparison tables.
Results
20 beams of SRS were audited. Measured and planned doses agreed within ±2% in the PTV. Gamma factor passing exceeded 90% for both spherical and irregular targets. Output factor analysis demonstrated systematic differences between flattening filter-free and flattened beams, particularly for field sizes below 2×2 cm². Geometric verification identified clinically relevant deviations in isocenter accuracy and imaging system reproducibility that would not be detected through remote audit methodologies.
Conclusion
This work demonstrates that an integrated on-site SRS audit provides clinically actionable information. By jointly evaluating geometric accuracy, small-field dosimetry, and volumetric dose delivery, the proposed framework enhances standardization, reduces clinical risk, and supports safe implementation of high-precision SRS.