(Thursday, 8/2/2018) 7:30 AM - 9:30 AM
Room: Karl Dean Ballroom A1
Purpose: To characterize the performance of a novel double-stack MLC.
Methods: The MLC system on the MRIdian MR-Linac (Viewray) consists of two stacks: upper layer with 34 leaf pairs and lower layer with 35 leaf pairs. The leaf width is 8.3mm when projected at the isocenter. The two layers are staggered to achieve an effective leaf width of 4.15mm. The RF shielding of the MLC motor and cable were evaluated with the ACR MRI phantom. The signal-to-noise ratios (SNR) were calculated and compared for cine images acquired with and without MLC motion. Leaf transmission was assessed with radiosensitive films for the scenario that the leaf ends of upper and lower layer aligned at the same location with leaves in each layer interdigitated. Leaf positioning accuracy was investigated separately for two layers forming picket-fence patterns. Output factors of MLC-defined fields were measured with an ionization chamber(standard imaging) for symmetric field sizes ranging from 2.49 to 24.07cm, and compared to the integrated Monte Carlo TPS calculated values.
Results: The average SNR for images with either stationary or moving MLC were comparable, with values of 109.9, 112.1, and 106.0 correspondingly. The intra- and inter-leaf transmission values are below the 1% IEC standard, but the maximum leakage through leaf-end was measured as 3.3%. The picket-fence tests indicated a 0.5 cm mis-calibration could be easily detected and the tests are recommended to be performed for each layer individually. The output factors agree with the TPS value within 1.5% for all measured field sizes.
Conclusion: The double-stack MLC system has adequate RF shielding, an overall less inter- and intra- leaf transmission, and a good agreement with TPS on output factor prediction. Further study is needed to investigate the leaf penumbra, output factor modeling for small fields, and MLC pattern reproducibility.
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