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What VMAT Delivery Errors Can Be Detected with Commercial Dosimetric Systems? - A Comparison of Three Dosimetric Systems

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L Holloway

S Arumugam1, T Young1, A Xing1, L Holloway1,2,3,4*, (1) Liverpool and Macarthur Cancer Therapy centres and Ingham institute, Sydney, NSW, (2) Institute of Medical Physics, University Of Sydney, Sydney, NSW, Australia (3) Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, Australia (4) University of New South Wales,Sydney, NSW, Australia

TU-C-108-9 Tuesday 10:30AM - 12:30PM Room: 108

Purpose: To study the sensitivity of three commercial dosimetric systems MULTIcube (MatriXX Evolution), ArcCHECK and Octavius-4D, in detecting errors during the delivery of Volumetric Modulated Arc Therapy (VMAT).

Methods:A simple VMAT arc with 111 Control-Points (CP) was created in Pinnacle using a 6MV photon beam model for an Elekta-Synergy accelerator. The aperture size for each CP was set to 5 x 10 cm2 and equal weights were assigned to all CPs. Three error scenarios considering Multi Leaf Collimator (MLC) position and dose delivery were simulated: Type1: Systematic shift in MLC position (from 1mm to 5mm) to the whole arc and systematic variation in output (from 1% to 10%) to the whole arc, Type2: Sag in MLC position as a function of gantry angle (amplitude ranging from 1mm to 5mm) and output variation as a function of gantry angle (amplitude ranging from 1% to 10%) , Type3: MLC and output errors (ranging from 1mm to 10mm and 1% to 100% respectively) introduced to only two of the CPs. The difference in gamma (γ) pass rate between the error-free and error-simulated measurements with 2%/2mm tolerance was calculated to assess the sensitivity of dosimetric systems in detecting errors.

Results:All three dosimetric systems detected type1 error. The γ pass rate reduced gradually as the error value increased beyond 2%/2mm tolerance. The MULTIcube system failed to detect the type2 errors whereas the γ pass rate in ArcCHECK and Octavius measurements decreased as the simulated error value increased. For type3 errors MULTIcube and Octavius measurements showed no change in pass rate whereas Arccheck measurements showed 5.9% and 2.6% decrease in pass rate at 10mm MLC error and 100% dose error respectively.

Conclusion:ArcCHECK was shown to be sensitive to all types of simulated error. MULTIcube showed the least sensitivity, detecting only systematic errors in whole arc

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