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Optimization of Electronic Portal Imaging Devices (EPIDs) for Small VMAT Fields

M Koo1*, J Darko2, E Osei3, (1) Ryerson University, Toronto, ON, CA, (2) Grand River Hospital, Kitchener, ON, CA, (3) Grand River Regional Cancer Centre,


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

Room: AAPM ePoster Library

Purpose: The purpose of this study is to evaluate and optimize the effectiveness of EPIDs for small field VMAT plans. This is accomplished through optimizing the PDIP algorithm for an extended SID.

Methods: Standard QA procedures at our institution uses a source-to-imager distance (SID) of 100 cm for all pre-treatment verification plans using a general purpose Portal Dose Image Prediction (PDIP) algorithm configured at this distance. A new PDIP algorithm was configured by generating a new kernel based on measured AIDA fields portal images and output factors measured at an SID=150cm. A small field PDIP algorithm was configured at SID=150cm and output factors for fields < 6x6cm. Four SRS brain cancer patient plans were randomly selected for evaluated with 3 scenarios: original general algorithm (SID=100cm,150cm), and new small-field algorithm (SID=150cm).

Results: In general, increased gamma passing rates were observed when portal images were normalized to the central axis compared to absolute dose. Greater gamma passing rates were also observed for measurements done at SID=150cm compared to SID=100cm for both clockwise and counterclockwise arcs. The new small field PDIP at SID=150cm provided the most improvement in gamma passing rates for either clockwise or counterclockwise arcs. Portal images acquired at SID=150cm inherently have an increased sensitivity due to the increased spatial resolution relative to the field portal at the extended SID.

Conclusion: Results from this study indicate that an optimal configuration geometry is necessary to optimize the pre-treatment QA of small field VMAT plans. An extended SID (such as SID=150cm) provides potential increase in sensitivity and may be more effective for evaluating pre-treatment quality assurance for small field VMAT plans. Further optimization of the SID may be required before implementation for any particular clinical use.


Quality Assurance, Small Fields, Portal Imaging


TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction

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