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Comparison Study for Portal Dosimetry Vs Diode Array in VMAT/IMRT Patient QA

N Demez*, D Soultan, R Badkul University of Kansas Hospital, Overland Park, KS

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The aim of this study to compare the quality assurance (QA) results between MapCHECK2 and Varian aS-100 portal dosimetry system for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) patient specific treatment plans.


Methods: A total of 10 IMRT and VMAT which were used for patient treatments were selected for this study. The IMRT and VMAT plans were delivered with Trubeam Linac. The Sun Nuclear MapCHECK 2 diode array with 10 cm water-equivalent build-up sleeve was positioned on treatment couch to collect measured values. The very same patients’ dose responses to The Varian TrueBeam PortalVision portal imager were computed with Varian portal dose image prediction (PDIP) module and delivered with same Linac after imager was configured and calibrated according to manufacturer’s instructions. The diode array was calibrated to scale 1 cGy to 1MU on 100 SAD while portal imager was scaled to have 1 (calibrated unit) CU for 1MU on 100 SSD. The gamma comparison test was utilized for both portal image and MapCHECK 2 results by using SNC patient software with 10 % threshold and 2%/2mm, 3%/3mm difference/distance-to-agreement (DTA) criteria.


Results: The average gamma passing rate is greater than 95% for both criteria and for both comparison methods. Both methods show comparable passing rate which is 95.6%±1.1% and 98.7%±1.4% for MapCHECK 2 and portal dosimetry respectively. Moreover, passing rates for lower criteria is somewhat better for portal dosimetry due to better resolution of portal imager. But for larger fields, MapCHECK 2 shows better performance than portal imager although the difference is small.


Conclusion: 2D diode array and portal imager can be used safely for pre-treatment verification. For smaller fields, portal imager shows better accuracy compared diode array.

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