The impact of MLC transmitted radiation on EPID dosimetry for dynamic MLC beams

Journal Article · Tue Apr 15 04:00:00 EDT 2008 · Medical Physics

DOI:https://doi.org/10.1118/1.2885368· OSTI ID:21120635

Vial, Philip; Greer, Peter B; Hunt, Peter; Oliver, Lyn; Baldock, Clive ^[1]

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1. Royal North Shore Hospital, Sydney (Australia)

The purpose of this study was to experimentally quantify the change in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to dynamic multileaf collimator (dMLC) beams with varying MLC-transmitted dose components and incorporate the response into a commercial treatment planning system (TPS) EPID prediction model. A combination of uniform intensity dMLC beams and static beams were designed to quantify the effect of MLC transmission on EPID response at the central axis of 10x10 cm{sup 2} beams, at off-axis positions using wide dMLC beam profiles, and at different field sizes. The EPID response to MLC transmitted radiation was 0.79{+-}0.02 of the response to open beam radiation at the central axis of a 10x10 cm{sup 2} field. The EPID response to MLC transmitted radiation was further reduced relative to the open beam response with off-axis distance. The EPID response was more sensitive to field size changes for MLC transmitted radiation compared to open beam radiation by a factor of up to 1.17 at large field sizes. The results were used to create EPID response correction factors as a function of the fraction of MLC transmitted radiation, off-axis distance, and field size. Software was developed to apply the correction factors to each pixel in the TPS predicted EPID image. The corrected images agreed more closely with the measured EPID images in areas of intensity modulated fields with a large fraction of MLC transmission and, as a result the accuracy of portal dosimetry with a-Si EPIDs can be improved. Further investigation into the detector response function and the radiation source model are required to achieve improvements in accuracy for the general case.

• OSTI ID: 21120635
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 4 Vol. 35; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

62 RADIOLOGY AND NUCLEAR MEDICINE
ACCURACY
BEAM PROFILES
COLLIMATORS
COMPUTER CODES
DOSIMETRY
IMAGES
RADIATION DOSES
RADIATION SOURCES
RESPONSE FUNCTIONS
SEMICONDUCTOR MATERIALS
SILICON