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Development of a High Resolution EPID Solution for Small Field Dosimetry

B Han

B Han1*, G Luxton1, S Yu1, M Lu2, L Wang1, E Mok1, L Xing1, (1) Stanford Univ School of Medicine, Stanford, CA, (2) PerkinElmer Imaging, Santa Clara, CA

SU-C-105-6 Sunday 1:00PM - 1:55PM Room: 105

Purpose: The increasing use of small beams in VMAT and SBRT presents significant challenges and calls for new tools for dosimetry measurements. The purpose of this study is to investigate a high spatial-resolution (0.2mm) amorphous silicon flat-panel electronic portal imaging device (EPID) for small field dosimetry.

Methods: A previously-developed EPID dosimetry system was used to measure the dose distribution of small fields from raw EPID-measured images. The response of the EPID specific to the linac was obtained by using a Monte Carlo simulation and a comprehensive calibration of the detector. The dosimetry system was validated against measurements of field sizes greater than 3x3cm2. In the present study, the EPID-based dosimetry measurement technique was used to obtain the relative output factor for small fields from 0.5cm to 3cm respectively created by both the jaws and the MLCs. 6MV, 10MV, and 15MV photon beams from a Varian TrueBeam were tested. The results were compared with measurements using EBT3 film, EDGE diode detector, and PinPoint ion chamber.

Results: The diode measurement was considered as a reference in this study based on the expectation that its small size(0.8mm) avoids the field size-dependent artifacts of larger detectors. For jaw-defined fields between 2 and 3cm, measurements with all detectors agreed with diode measurements to within 2.7%. For jaw field sizes <2cm, the relative output factors measured using the EPID, film and PinPointTM were lower than the diode by averages of 3.5%, 15.1% and 23.1%, respectively. For MLC fields, output factors measured with EPID, film and PinPoint were found to differ from the diode measurements by averages of +1.7%, -2.2% and -8.1%, respectively.

Conclusion: The high spatial resolution EPID dosimetry system proved to be an accurate and efficient dosimetric tool for small field measurements. Accurate output factors can be measured for independent dosimetry calibration and verification.

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