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Performance of a Multi Leaf Collimator System On 1.5T MR-Linac

K Zhang*, Y Tian, K Men, J Dai, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital,Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, Beijing, CN,


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

Room: AAPM ePoster Library

Purpose: To evaluate the performance of Agility multileaf collimator (MLC) in the high-magnetic-field of Elekta Unity MR linac.

Methods: The MLC motor RF shielding was evaluated using homogeneous phantom with real-time imaging and quantified by signal-to-noise ratio (SNR). Field of 200 x 200mm was measured for calculating leaf width. Output of 100 x 100mm fields at center and off-axis positions were used for determining leaf transmission and profiles for field penumbra. Output was measured with PTW TW31021 ion chamber. While profiles were measured with microDiamond detector and BEAMSCAN® MR water phantom. A 190 x 220 mm field composed of two strip segments was exposed on EBT3 film to assess tongue-and-groove effect, and the minimal gap fields were also exposed on EBT3 film at center and off-axis position to assess leakage between leaf tips in the closed position. Picket fence test with EPID was performed for analyzing leaf positional reproducibility.

Results: Leaf motion leads to small fluctuation for transverse plane’s SNR with thorax and abdomen bFFE images. Leaf width is 7.1mm. The leaf transmission is 1.09% and reduced to 0.85% when diaphragm blocks the field with minimum MLC gap. The maximum transmission through both the diaphragm and MLC leaves is 0.18%. Leaf penumbra ranges from 5.75 to 6.98 mm in the GT direction and from 6.52 to 7.7 in the AB direction. The tongue-and-groove effect causes an underdose up to 27%. Leaf tip leakage improved from 60% at field center to 3% at 10cm off-axis. Average leaf positioning error is -0.01mm for all leaves based on picket fence test in two months.

Conclusion: The Agility MLC has a good RF noise shielding design, low radiation leakage, comparable tongue-and-groove effect and high positioning reproducibility on the Elekta Unity MR linac.

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Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Natural Science Foundation of China (11875320).


MLC, Performance Tests, MR


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

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