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Program Information

A Novel Quality Control Test for Gantry Angle Accuracy in Respiratory-Gated VMAT Treatments

E Hubley

E Hubley*, M Briscoe , N Ploquin , G Pierce , Tom Baker Cancer Center, Calgary, AB


SA-B-BRA|B-5 (Saturday, March 18, 2017) 10:30 AM - 12:30 PM Room: Ballroom A|B

Purpose: The dosimetric accuracy of respiratory-gated volumetric modulated arc therapy (VMAT) relies on the mechanical accuracy of the gantry position as it stops, rewinds and re-builds inertia between each gating window. A novel quality control (QC) test was developed to identify any errors in gantry angle position; the sensitivity of this test was determined by simulating geometric errors of different sizes.

Methods: The QC and sensitivity testing treatment plans were created in Eclipse™. The collimator was set at 90°, the y-jaws each at +1cm, and the x-jaws at +5cm, leaving a 2cm wide and 10cm long slit. The QC plan delivered 10MU/° through the slit at a continuous dose rate and gantry speed over 360° and was delivered with and without gating. Sensitivity testing plans used the same jaw positions and delivered 10MU/° over 30° arcs with arcs overlapped and underlapped by 0.5, 1, 2, 3, and 5° to simulate corresponding gantry angle errors in gated delivery. 20cm x 5cm strips of Gafchromic® EBT3 film were taped to the surface of the Catphan® 504 phantom, mounted at isocentre, and covered with 1cm of bolus. The sensitivity arcs were delivered to the film, and the film was scanned. Doselab Pro Software (v6.8) was used to convert to optical density and take a profile through the film. The size of the overlaps and underlaps visible in the dose profiles were quantified by integrating in Python.

Results: In Doselab Pro, errors corresponding to gantry angle overlaps and underlaps of 1° were easily detectable on the film. When delivering the QC plan, no differences were detectable between the deliveries with and without gating, and no gantry angle errors were detected.

Conclusion: A QC test was developed to identify errors as small as 1° in gantry angle position during gated RapidArc® delivery.

Funding Support, Disclosures, and Conflict of Interest: Funding Support: Enhanced Care Grant: Consistent Lung, Liver, and Spine SBRT for Albertans

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