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.