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Dosimetric Leaf Gap and Transmission for Millennium 120-Leaf MLC On Varian TrueBeam

Johnson Darko1,2,3*, Patricia Bui2 , Dinindu Gunasekara2 , Andre Fleck1,3 , Ernest Osei1,2,3 , (1) Grand River Regional Cancer Centre, Kitchener, ON, (2) University of Waterloo, Waterloo, ON, (3) University of Guelph, Guelph, ON


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: In external beam radiotherapy, measured parameters such as the Dosimetric Leaf Gap (DLG) and Multi Leaf Collimator (MLC) transmission used for dose calculation can introduce small dose errors if optimal values are not used. When treating with a modulated high dose in relatively small fields, these minor errors play a major role in the overall uncertainty in the treatment delivery. The aim of this study is to determine the optimal value for the DLG and MLC transmission using various detector systems.

Methods: DLG and MLC transmission were measured for photon beam energies: 6MV, 6MV FFF, 10MV, 10MV FFF, and 15MV on three Varian TrueBeamâ„¢ linacs equipped with Millenium 120-leaf MLC. Two ion chambers: PTW 0.6cc farmer chamber and Exradin A14 microchamber, and Electronic Portal Imaging Device (EPID) were used for all measurements. The measured data were further validated using Exradin W1 Scintillator and Gafchromic films. The transmission through the MLC was measured by delivering 2000 monitor units (MU) with a field size of 10 cm x 26 cm and with the MLC closed underneath the jaw. The DLG was measured using 100 MU and a modified Varian sliding slit technique.

Results: The measured MLC transmission varied within 0.5% between all of the detectors. There was also no statistical significance in MLC transmission between the leaf banks and between the machines. The DLG was found to be both field size and energy dependent. The DLG value increases with increasing field size and energy.

Conclusion: Accurate measurement of MLC transmission and DLG values play an important role in modelling dose in the Eclipse TPS and are dependent of the choice of detector system used for the measurement.


MLC, Treatment Planning, Transmission Measurements


Not Applicable / None Entered.

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