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Measurement Based Dose-Volume Metrics for the Quality Assurance of VMAT Plans -Are We There Yet?

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S Arumugam

S Arumugam1,2*, A Xing1,2 , T Young1,2 , S Blake2,3 , D Thwaites3 , L Holloway1,2,3,4 , (1) Liverpool and Macarthur Cancer Therapy Centres, Sydney, NSW,Australia, (2) Ingham Institute, Sydney, NSW,Australia, (3) Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW, Australia,(4)Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, Australia


SU-F-BRB-9 (Sunday, July 12, 2015) 4:00 PM - 6:00 PM Room: Ballroom B

Purpose: To validate ArcCHECK measurement-based 3DVH dose-volume metrics for use with volumetric modulated arc therapy (VMAT) plans.

Methods: Five each of prostate and head and neck (H&N) VMAT plans were considered for this study. Three types of errors were introduced into the original plans: gantry angle independent MLC shift (ranging from -5mm to 5mm), gantry angle dependent MLC error (ranging from 1mm to 7mm) and gantry angle dependent dose error (ranging from 1% to 10%). The percentage difference in PTV-D95 as estimated by 3DVH for the error-introduced delivery was compared with a similar metric calculated by comparing the dose matrices of the error-free and error-introduced plans calculated in the TPS. The gamma analysis with 2%L/2mm and 3%L/3mm tolerance criteria was also calculated by comparing the dose matrices.

Results: For MLC shift errors, the TPS dose matrix comparison showed that the PTV-D95 for the prostate plans reduced by a maximum (SD) of -8.4(3.3)% for the 5mm error. The 3DVH-estimated D95 reduced from -0.5(2.0) for no-error plans to -1.0(3.3)% for the 5mm MLC shift. Similarly for H&N plans the TPS-calculated PTV-D95 reduced by -5.1(3.8)%, with the 3DVH-estimated D95 reducing from -1.5(2.2)% to -3.2(5.8)%. For gantry dependent MLC errors, the TPS D95 of prostate plans reduced to a maximum of -14.1(8.5)%, while the 3DVH-estimated D95 reduced to 0.6(4.2)%. For H&N plans the TPS D95 reduced to -1.7(1.7)%, while the 3DVH-estimated D95 reduced to -4.0(4.1)%. No major change in D95 was observed for gantry dependent dose errors for the TPS or 3DVH. In all error scenarios the gamma pass rate decreased as the magnitude of error increased.

Conclusion: The gamma comparison of dose matrices was shown to detect all type of errors considered. The 3DVH-estimated D95 agreed with the TPS more closely in H&N plans than prostate plans.

Funding Support, Disclosures, and Conflict of Interest: This project was funded through a Cancer Council NSW Project Grant (RG14-11)

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