Effect of Gantry Angle On Portal Dosimetry Vs Ion Chamber Array of Linac Based IMRT QA
S Wang1*, S Gill2, R Wynn3, M Huq4, S Wang5, (1) UPMC Hillman Cancer Center at Erie, Erie, PA, (2) University of Pittsburg Medical College UPMC Hillman Cancer Center, Erie, PA, (3) Loyola University Chicago, IL, (4) UPMC Hillman Cancer Center and University of Pittsburgh School of Medicine, Pittsburgh, PA, (5) UPMC Hillman Cancer Center at Erie, Erie, PA
Presentations
PO-GePV-P-48 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Comparison of QA techniques for accelerated partial breast irradiation (APBI) IMRT using EPID and ion chamber based 2D array and to verify the response electronic portal imaging device (EPID) and 2D array with respect to treatment gantry angle vs gantry zero.
Methods: Currently, all static IMRT QA plans are delivered at gantry angle zero. For APBI, comparison of patient QA between 2D array (Matrixx) and Linac based Portal Dosimetry QA (PDQA) was performed. QA plans were created at gantry angle of zero degree as well as planned gantry angles for both PDQA as well as Matrixx. QA plans were delivered on the Varian linac. Gamma analysis for each field and for composite plan was compared between PDQA and 2D array at both gantry zero and planned gantry angles. Gamma analysis for PDQA was analyzed using Varian Portal Dosimetry software and for Matrixx using OmniPro IMRT QA software.
Results: 2D Gamma analysis results were analyzed for Portal Dosimetry and for Matrixx. Portal Dosimetry at gantry zero (2%/2mm Gamma criteria) resulted in fields 1 to field 5 are 96.8%; 94%, 96.5 %, 93.4% and 92.2% of the pixels passing Gamma criteria, respectively. Composite plan 2%/2mm Gamma analysis is 88.6%. Gamma analysis at planned gantry angles are: 97.5%, 95.1%, 96.9%; 94.8% and 93.5%, respectively. Composite plan result is 92.5%. For Matrixx Gamma analysis for composite plan for gantry zero is 89.37%. Composite at planned gantry angles is 89.16%.
Conclusion: Both portal imager and 2D ion chamber array can be used for APBI IMRT QA. QA results are better with portal imager as compared to 2D array. However, portal imager results show more variation with gantry angle vs gantry zero. Gamma analysis results at planned gantry angle for Portal Dosimetry are better than gantry zero for both field-by-field analysis and composite plan analysis.
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