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Automated Picket Fence Test for Full MLC Bank On the Elekta Unity

D Dunkerley*, A Boczkowski, S Strand, T Geoghegan, J St-Aubin, S Yaddanapudi, J Snyder, D Hyer, University of Iowa, Iowa City, IA


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

Room: AAPM ePoster Library

Purpose: Elekta Unity MRI-Linac presents unique challenges to typical MLC quality assurance (QA) procedures due to the closed-bore design (70cm diameter), which limits the user from positioning large measurement equipment at isocenter, combined with the 57.4 cm wide radiation field made up of 160 MLC leaves. The purpose of this work is to develop an automated procedure for performing QA on the full MLC bank.

Methods: picket fence field was created with the linac positioned at a gantry angle of 315° to avoid passing through the treatment table and cryostat pipe. Four, 8mm wide open strips were positioned such that a 2mm gap was present between each strip, resulting in 3 underexposed strips centered at positions of -1, 0, and +1cm from the center of the MLC bank. In order for the 57.4cm wide field to fit on a single piece of film, a jig was manufactured to position a piece of 32.5 by 43.2cm EBT3-1417 radiochromic film oriented with the radiation exposure along the diagonal axis. Additionally, the jig held the film at a distance of 123.3cm from the source to minimize the field size. During irradiation, the film was positioned between 2mm thick pieces of copper to provide buildup and mitigate the electron return effect. The film was digitized using an Epson 12000XL scanner at 75 DPI and processing was automated using in-house developed software using MATLAB to identify each leaf position and the location of the 2mm underexposed strips from the resulting profiles.

Results: MLC leaves were able to be analyzed from film measurements with measured positions falling within 0.01±0.19mm from the expected positions and a maximum deviation of 0.48mm.

Conclusion: our knowledge, this is the first quantitative MLC QA solution for the Elekta Unity that evaluates the entire bank of MLC leaves.




TH- External Beam- Photons: Quality Assurance - Linear accelerator

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