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

Abstract

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). Performance was evaluated as recommended in AAPM Report Nos. 72 and 142, and MLC motor radiofrequency (RF) shielding was also evaluated. Fields of 99.8×99.8mm2 and 199.6×199.6mm2 were examined to calculate the leaf width. Profiles of 28.4×28.4mm2 , 57.0×57.0mm2 , 99.8×99.8mm2 , and 199.6×199.6mm2 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 190.0×220.0mm2 field composed of two strip segments to assess the tongue-and-groove (T&G) effect. Outputs of 100.0×100.0mm2 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). The leaf widths were 7.00±0.00 and 7.10±0.00mm for 99.8×99.8 and 199.6×199.6mm2 fields, respectively. The leaf side penumbra ranged from 5.11±0.08 to 11.28±0.29mm and the leaf tips penumbra ranged from 4.47±0.03 to 6.25±0.15mm for the field size range of 28.4×28.4mm2 to 199.6×199.6mm2 . 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 100mm off-axis. At 6months, the short-term leaf position reproducibility was 0.11mm (2σ), and the long-term leaf position reproducibility was 0.19mm (2σ). Leaf motion led to small fluctuations of the SNR of up to 1.64%. 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.