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Spatial Variation of MLC Dosimetric Leaf Gap and Transmission Factor for Flattened and Unflattened Beams On TrueBeam and Edge Using DMI Panels and Developer Mode

D Barbee

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


TH-AB-FS1-13 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 1

Purpose: Dosimetric leaf gap (DLG) and transmission factor (TF) are known to vary spatially for flattened beams, but have not been characterized for unflattened (FFF) beams on newer TrueBeam machines. Varian’s new a-Si1200 digital megavoltage imager (DMI) has been designed for measurement of FFF beams. This work investigates DMI acquired DLG and TF maps for standard and FFF beams on TrueBeam and Edge machines with comparison to other measurement techniques.

Methods: Varian provided DLG DICOM plans were individually converted to Developer Mode XML files then combined into a single XML with MV Dosimetry image acquisitions added for each sliding gap. Y jaws were adjusted to include all MLCs. The Developer Mode file was delivered on 4 TrueBeam and 1 Edge machines equipped with DMI panels, using 6X, 6X-FFF, and 10X-FFF energies. Pixel-wise DLG and TF processing was performed in Matlab. The same Developer Mode file was used to expose an A12 ion chamber and water phantom at 95cm SSD, 5cm depth as well as SNC MapCHECK2 for comaprison.

Results: Acquisition of all DLG and TF images in Developer Mode took approximately 3.6, 2.8, and 2.0 minutes for 6X, 6X-FFF, and 10X-FFF energies, respectively. DLG modulation varied by 0.25-0.5 mm between intraleaf/interleaf positions for all energies. DLG remained relatively consistent across all leaves for FFF beams, but decreased by 0.4 mm at 20 cm from CAX for 6X. DMI TF measurements matched A12 measurements well with the exception of 6X-FFF, which was underestimated by 0.3%. All DMI DLGs were underestimated in comparison to MapCHECK2 and A12 measurement, particularly 6X-FFF by approximately 50%.

Conclusion: Developer mode and DMI panels provide tools for spatial investigation into dosimetric leaf properties. Absolute DLGs are underestimated, but relative trends match to MapCHECK2 well. DMI energy dependence should be investigated for TF measurement to improve dosimetric results.

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