Program Information
EPIQA as Patient Specific Quality Assurance Tool for VMAT
V Subramani1,2*, M Radhakrishnan2 , A Nambiraj2 , M Manavalan1 , S Chitra1 , M Venkataraman1 , (1) Apollo Cancer Institutes, Chennai, Tamil Nadu, (2) VIT University, Vellore, Tamil Nadu
Presentations
SU-I-GPD-T-263 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To evaluate the patient specific quality assurance (PSQA) results of Volumetric Modulated Arc Therapy (VMAT) treatments done using EPIQA.
Methods: EPIQA, which converts an acquired portal image into measured dose and comparing it with TPS calculated dose is used as PSQA tool for 160 VMAT treatment plans belonging to different treatment sites viz brain, head & neck, thorax, abdomen and pelvis. The PSQA treatment plans are delivered at the linac and integrated images are acquired using Electronic Portal Imaging Device (EPID) for analysis in EPIQA. The Gamma Agreement Index (GAI) is evaluated and studied for various gamma criterias Distance To Agreement (DTA) / Dose Difference (DD using both global and local gamma evaluation methods for individual patient treatment sites.
Results: The average global GAI & local GAI including all treatment sites are 97.4±0.6% & 92.3±1.2%, 94.0±1.3% & 89.0±1.4%, 87.5±1.9% & 83.3±1.1%, 96.0±0.7% & 86.7±1.6%, 88.7±3.3% & 81.7±2.6%, 88.6±2.5% & 74.0±3.3%, and 64.0±3.0% & 51.9±2.7% for 3mm/3%, 3mm/2%, 3mm/1%, 2mm/3%, 2mm/2%, 1mm/3% and 1mm/1% respectively. The change in GAI is more sensitive with DTA than with DD and the gamma passing rate is poor (<80%) for 1mm DTA used. The reason may be attributed for not including the gantry position dependent EPID imager sag in correcting the acquired integrated image for VMAT treatments. The GAI shows strong positive correlation (Pearson’s r >0.9) with both DTA and DD as well irrespective of the selected patient treatment site and selected analysis method. Global GAI is statistically high (>5%) than the local GAI and the difference is pronounced in pelvic plans where points failing the local gamma criteria were mostly in lower dose and low gradient regions because of the possible deficiency in commissioning TPS or EPIQA in modeling low dose regions.
Conclusion: The PSQA of VMAT treatments are evaluated successfully using EPIQA.
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