Performance of a multileaf collimator system for a 1.5T MR‐linac.

Purpose: The purpose of this study was to evaluate the performance of a multileaf collimator (MLC) system for the high magnetic field of the Elekta Unity magnetic resonance linear accelerator (MR‐linac). Methods: Performance was evaluated as recommended in AAPM Report Nos. 72 and 142, and MLC motor radiofrequency (RF) shielding was also evaluated. Fields of 99.8 × 99.8 mm2 and 199.6 × 199.6 mm2 were examined to calculate the leaf width. Profiles of 28.4 × 28.4 mm2, 57.0 × 57.0 mm2, 99.8 × 99.8 mm2, and 199.6 × 199.6 mm2 fields at center and off‐axis positions were measured using a microDiamond detector and BEAMSCAN® MR water phantom to determine the field penumbra. EBT3 film was exposed to a 190.0 × 220.0 mm2 field composed of two strip segments to assess the tongue‐and‐groove (T&G) effect. Outputs of 100.0 × 100.0 mm2 open and blocked fields at center and off‐axis positions were measured using a PTW TW31021 ion chamber and a film with the full MLC‐blocked field to determine transmission. The minimum gap fields were also exposed on EBT3 film at center and off‐axis positions to assess leakage between leaf tips in the "closed" position. The picket fence test was performed using a megavoltage imager to analyze leaf positional reproducibility. RF shielding was measured via real‐time MR imaging of a homogeneous phantom and quantified using the signal‐to‐noise ratio (SNR). Results: The leaf widths were 7.00 ± 0.00 and 7.10 ± 0.00 mm for 99.8 × 99.8 and 199.6 × 199.6 mm2 fields, respectively. The leaf side penumbra ranged from 5.11 ± 0.08 to 11.28 ± 0.29 mm and the leaf tips penumbra ranged from 4.47 ± 0.03 to 6.25 ± 0.15 mm for the field size range of 28.4 × 28.4 mm2 to 199.6 × 199.6 mm2. The T&G effect resulted in an underdose of up to 23.41 ± 0.88%. The max leaf transmission was 0.33 ± 0.00% when MLCs were all closed and diaphragms were all opened, and this value was reduced to 0.18 ± 0.00% when the diaphragm blocked the field with a minimum MLC gap. The leaf tip leakage rates were 60.08 ± 0.05% at field center and 2.94 ± 0.00% approximately 100 mm off‐axis. At 6 months, the short‐term leaf position reproducibility was 0.11 mm (2σ), and the long‐term leaf position reproducibility was 0.19 mm (2σ). Leaf motion led to small fluctuations of the SNR of up to 1.64%. Conclusion: The new MLC system has a comparable T&G effect, low radiation leakage, high positioning reproducibility, and good RF shielding for the Elekta Unity MR‐linac. However, more attention is required for the positional change of the leaf side penumbra. [ABSTRACT FROM AUTHOR]