Program Information
Filmless Treatment Localization QA for the CyberKnife System
J Gersh1,2*, M Noll3 , (1) Gibbs Cancer Center & Research Institute - Pelham, Greer , SC, (2) Spectrum Medical Physics, LLC, Greenville, SC, (3) Accuray Incorporated, Sunnyvale, CA
Presentations
TH-AB-201-7 (Thursday, August 4, 2016) 7:30 AM - 9:30 AM Room: 201
Purpose: Accuray recommends daily evaluation of the treatment localization and delivery systems (TLS/TDS) of the CyberKnife. The vendor-provided solution is a Winston-Lutz-type test that evaluates film shadows from an orthogonal beam pair (known as AQA). Since film-based techniques are inherently inefficient and potentially inconsistent and uncertain, this study explores a method which provides a comparable test with greater efficiency, consistency, and certainty. This test uses the QAStereoChecker (QASC, Standard Imaging, Inc., Middleton, WI), a high-resolution flat-panel detector with coupled fiducial markers for automated alignment. Fiducial tracking is used to achieve high translational and rotational position accuracy.
Methods: A plan is generated delivering five circular beams, with varying orientation and angular incidence. Several numeric quantities are calculated for each beam: eccentricity, centroid location, area, major-axis length, minor-axis length, and orientation angle. Baseline values were acquired and repeatability of baselines analyzed. Next, errors were induced in the path calibration of the CK, and the test repeated. A correlative study was performed between the induced errors and quantities measured using the QASC. Based on vendor recommendations, this test should be able to detect a TLS/TDS offset of 0.5mm.
Results: Centroid shifts correlated well with induced plane-perpendicular offsets (p < 0.01). Induced vertical shifts correlated best with the absolute average deviation of eccentricities (p < 0.05). The values of these metrics which correlated with the threshold of 0.5mm induced deviation were used as individual pass/fail criteria. These were then used to evaluate induced offsets which shifted the CK in all axes (a clinically-realistic offset), with a total offset of 0.5mm. This test provided high and specificity and sensitivity.
Conclusion: From setup to analysis, this filmless TLS/TDS test requires 4 minutes, as opposed to 15-20 minutes for film-based methods. The techniques introduced can potentially isolate errors in individual joints of the CK robot.
Funding Support, Disclosures, and Conflict of Interest: Spectrum Medical Physics, LLC of Greenville, SC has a consulting contract with Standard Imaging of Middleton, WI
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