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Array Measurement and Evaluation of 2D Dosimetric Leaf Gap of a MLC System

T Chen*, J Xue, D Barbee, New York University, New York, NY

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To analyze and quantify physics factors that impact the local dosimetric leaf gap (DLG), which is a strongly contributing factor to failed PSQA for intensity modulated plans, of the STD120 MLC installed on a Varian TrueBeam unit, and reconstruct 2D DLG maps for the MLC at multiple photon energies.
Methods: Multiple xml Varian Developer Mode files for DLG measurement were developed based on the Varian standard sweeping gap technique, including: expanded Y-jaw to 36cm, increased MUs (improved SNR), and MLC and x-jaws shift 3cm laterally (off axis DLG measurement). SunNuclear MapCheck2 (diodes, primary detector) and Ashland GafChromic3 (film, secondary detector) were used to measure MLC transmission and dosimetric response of each sweep gap separately. Measurements were made at 100cm source to detector distance with 5cm buildup. For MapCheck2, all measurements were repeated with a 2.5mm IEC+Y shift to account for MLC tongue and groove effects. DLGs were calculated using the extrapolation method and compared against clinical baseline.
Results: The DLG derived from the array measurements showed variations between leaf widths, leaf gap, and distance from central axis. 5mm leaf DLGs were 26.8%, 21.6%, 26.5%, and 18.2% larger than 1cm leaf DLGs under 6X, 6FFF, 10FFF, and 15X beams, respectively. MapCheck2 measured DLGs under the leaf gap (no shift) were 16.8%, 17.7%, 20.3%, and 19.3% higher than DLG under the leaf (measured with 2.5mm shift) for 6X, 6FFF, 10FFF, and 15X beams, respectively. Average leaf gap and leaf DLGs were within 1% of planning system values for all energies. DLGs under the same leaf showed a slight decrease with increasing central axis distance (0.005mm/cm).
Conclusion: The proposed work showed that the DLG values can vary more than 15% based on the position of measurement. 2D DLG map maybe necessary for a more accurate estimation on the dosimetry impact of DLG.

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