Room: Stars at Night Ballroom 1
Purpose: To evaluate the stability of daily output, beam energy, precision, and accuracy of a newly-installed, self-shielded ZAP-X (Zap Surgical, Inc.) SRS system.
Methods: Stability, precision, and accuracy were evaluated after acceptance testing and commissioning through measurements of daily output, beam energy, Winston-Lutz (WL), end-to-end (E2E), and patient-specific quality assurance (PSQA) over three months. ZAP-X output was calibrated to deliver 1.0 cGy/MU at dmax (7mm). ZAP-X system consists of a rotating non-coplanar 2.7MV photon beam designed to deliver isocentric treatments for intracranial tumor. Daily output was measured with a PTW 31010 (0.125cc) ion chamber (IC) at isocenter with a 25mm circular field size. Beam energy was calculated by taking the ratio of measurements at depths of 20cm and 10cm. Coincidence of radiation and imaging isocenters was performed by WL test. Precision and accuracy of overall treatment delivery was evaluated by E2E tests. PSQA was performed on five patients with EBT3 film in a Cyberknife head phantom and with a PTW pinpoint IC in a Lucy phantom.
Results: Output variation was bounded between Â±3% (29/31), and between Â±4.5% (2/31) with mean (1.003Â±0.021). Relative changes in beam energy ranged from (0.993-1.005) with mean (1.000Â±0.003). WL tests resulted with mean error of (-0.12Â±0.22), (0.19Â±0.19) and (0.23Â±0.21) mm in the X(L-R), Y(S-I) and Z(A-P) directions, respectively. Overall targeting error with WL was (0.45Â±0.15)mm. E2E tests using 36 beams resulted in mean errors of (-0.07Â±0.31), (0.18Â±0.35), and (-0.32Â±0.26)mm in the left, superior, and anterior directions, respectively. Overall average targeting error from E2E tests was (0.56Â±0.32)mm. PSQA results from our first five patients shows an average gamma passing rate of 97.83% (range: 94.00%-99.93%) at (2%, 2mm). On average, IC and TPS doses agreed to within 0.76% (range: -0.97%-2.84%).
Conclusion: The stability, precision and accuracy of the ZAP-X SRS system is adequate and acceptable for clinical use.