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Small Field Output Factor Measurement Using A-Si1200 DMI Panel

D Barbee

D Barbee*, I Das , NYU Langone Medical Center, New York, NY


SU-I-GPD-T-554 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: One of the most challenging aspects of small field dosimetry is the measurement of output factors (OF). A multitude of detectors for measurement exist, each with varying response, resolution, and intricacies for measurement (e.g. positioning). The work investigates the use of the new Varian a-Si1200 Digital Megavoltage Imager (DMI) for small field OF measurement.

Methods: Output factors were measured on a Varian Truebeam for 6X, 6X-FFF, 10X-FFF, 10X, and 15X energies for symmetric field sizes ranging from 0.5x0.5 to 10x10cm² using the following detectors for comparison to DMI: Standard Imaging Scintillation (W1), Sun Nuclear Edge, and PTW Diamond. Detector measurements were performed in water at a depth of 5cm with an SSD of 95cm, following Varian Eclipse beam data recommendations. DMI OF measurements were performed in Developer (Research) mode using custom XML files with the panel in Dosimetry acquisition positioned at isocenter. The effects of build-up, latent residual images (ghosting), and MU linearity were investigated using additional Developer mode XMLs. Resultant DICOM images were processed in Matlab.

Results: Output factors were compared to W1 scintillator based on its IAEA TRS 483 performance. Diamond and Edge output factors measured within +/-2% for field sizes greater than 2cm before climbing to approximately +5% at 0.5cm². DMI output factor XMLs were generated to mitigate the effects of linearity and ghosting of DMI measurements. DMI OFs with and without 5cm build-up consistently measured 5-10% below and 5-10% above reference OFs for all energies, respectively, both increasing by 5% going from 1x1cm² to 0.5x0.5cm².

Conclusion: Small field OF measurement using DMI allow for high resolution measurements of very small field sizes, but requires a combination of setup optimization and image processing. Measurement on the DMI could provide availability to measure small field OF for centers with limited access to high resolution detectors.

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