Geometric and radiation characterization of a new double stack multileaf collimator in a low field MR-Linac.
Doemer A, Glide-Hurst C, Kim J, Wen N, Movsas B, Chetty IJ. Geometric and radiation characterization of a new double stack multileaf collimator in a low field MR-Linac.. Med Phys 2018; 45(6):e144.
Purpose: With real-time imaging capability and double stacked, doublefocused MLCs, the new low-field MR-linac is well-suited for SBRT treatments. However, the MLC performance must be well-characterized to facilitate high precision radiation therapy. This work evaluates the MR-Linac MLCs geometric and radiation characteristics. Methods: MLC field sizes (FS) range from 0.2×0.4 cm2 to 27.4×24.1 cm2. Each leaf is 8 mm wide with one stack offset by 4 mm, producing 4 mm aperture resolution without the tongue/groove that is observed on typical single-stack MLCs. Transmission was measured using an MR-compatible A12 ion chamber (IC) and Gafchromic film. Picket fence (PF) centering accuracy and reproducibility was performed using Gafchromic film at cardinal gantry angles. FS accuracy and penumbra measurements for FS less than 1.66 cm2 were performed in a water tank with a diode detector, and for FS from 4.15 cm2 to the maximum FS with an IC array. Penumbra results were compared to the Monte Carlo-based treatment planning system. Longitudinal MLC positional accuracy was assessed via PF tests and FS measurements run over 8 months of clinical operation. Results: The IC measured leakage was 0.12%, and a maximum point leakage of 2.62% was measured on film. Centering error was consistent with repeated testing (-0.07 ± 0.17 mm (maximum 0.25 mm) and 0.03 ± 0.21 mm (maximum = 0.38 mm)) at gantry = 0. The difference between nominal and measured FS was 0.20 ± 0.40 mm (maximum = 0.26 mm) and 0.05 ± 0.30 mm (maximum = 0.37 mm) in crossplane and inplane directions, respectively. Field penumbra differences were 0.10 ± 0.40 mm (maximum = 0.84 mm) and 0.30 ± 0.24 mm (maximum = 0.89 mm) for crossplane and inplane, respectively. PF and FS measurements during routine QA showed continued MLC accuracy within TG- 142 guidelines. Conclusion: MLC measurements met AAPM TG-50, TG- 53, and TG-142 criteria and agreed within 1%-2% of TPS calculations. Future applications include stereotactic radiosurgery, where precise and accurate MLCs are of paramount importance.