Machine performance and stability of the first clinical self-shielded stereotactic radiosurgery system: Initial 2-year experience

This study provides insight into the overall system performance, stability, and delivery accuracy of the first clinical self-shielded stereotactic radiosurgery (SRS) system. Quality assurance procedures specifically developed for this unit are discussed, and trends and variations over the course of 2-years for beam constancy, targeting and dose delivery are presented. Absolute dose calibration for this 2.7 MV unit is performed to deliver 1 cGy/MU at d_max = 7 mm at a source-to-axis-distance (SAD) of 450 mm for a 25 mm collimator. Output measurements were made with 2-setups: a device that attaches to a fixed position on the couch (daily) and a spherical phantom that attaches to the collimating wheel (monthly). Beam energy was measured using a cylindrical acrylic phantom at depths of 100 (D₁₀ ) and 200 (D₂₀ ) mm. Beam profiles were evaluated using Gafchromic film and compared with TPS beam data. Accuracy in beam targeting was quantified with the Winston-Lutz (WL) and end-to-end (E2E) tests. Delivery quality assurance (DQA) was performed prior to clinical treatments using Gafchromic EBT3/XD film. Net cumulative output adjustments of 15% (pre-clinical), 9% (1st year) and 3% (2nd year) were made. The mean output was 0.997 ± 0.010 cGy/MU (range: 0.960-1.046 cGy/MU) and 0.993 ± 0.029 cGy/MU (range: 0.884-1.065 cGy/MU) for measurements with the daily and monthly setups, respectively. The mean relative beam energy (D₁₀ /D₂₀ ) was 0.998 ± 0.004 (range: 0.991-1.006). The mean total targeting error was 0.46 ± 0.17 mm (range: 0.06-0.98 mm) for the WL and 0.52 ± 0.28 mm (range: 0.11-1.27 mm) for the E2E tests. The average gamma pass rates for DQA measurements were 99.0% and 90.5% for 2%/2 mm and 2%/1 mm gamma criteria, respectively. This SRS unit meets tolerance limits recommended by TG-135, MPPG 9a., and TG-142 with a treatment delivery accuracy similar to what is achieved by other SRS systems.