Characterization of positional accuracy of a double‐focused and double‐stack multileaf collimator on an MR‐guided radiotherapy (MRgRT) Linac using an IC‐profiler array

Abstract

Purpose With advance magnetic resonance (MR)‐guided online adaptive radiotherapy (MRgoART) relying on calculation‐based intensity‐modulated radiation therapy (IMRT) quality assurance (QA), accurate and sensitive QA of the multileaf collimator (MLC) becomes an increasingly essential component for routine machine QA. As such, it is important to assure compliance with the AAPM TG142 guidelines to supplement calculation‐based QA methods for an online adaptive radiotherapy program. We have developed and implemented an efficient and highly sensitive QA procedure using an ionization chamber profiler (ICP) array to enable real‐time characterization of the positional accuracy of a double‐focused and double‐stacked MLC on a clinical MR‐guided radiotherapy (MRgRT) system and to supplement calculation‐based QA for an MRgoART program. Methods An in‐house MR‐compatible jig was used to position the ICP (detector resolution 5 mm on X/Y axis) at an extended SDD of 108.4 cm to enable each MLC leaf (8.3 mm leaf width at isocenter) to be uniquely determined by two neighboring ion chambers. The MRgRT linac system utilizes a novel jawless, double‐focused, and double‐stacked MLC design such that the upper bank (MLC1) and lower bank (MLC2) are offset by half a leaf width. Positional accuracy was characterized by three methods: single bank half‐beam block (HBB) at central axis, forward slash diagonal (FSD), and backslash diagonal (BSD) at off‐axis. Measurements were performed for each bank in which each leaf occludes half of a detector. A corresponding reference field with the MLC retracted from occlusion was measured. The sensitivities of HBB, FSD, and BSD were evaluated by introducing 0.5–2.5 mm of known errors in 0.5 mm increments, in both positive and negative directions. The relationship between detector response and MLC error was established. Over a 6‐month longitudinal assessment, we have evaluated MLC performance with weekly QA of HBB among cardinal angles, and monthly QA of FSD and BSD. Results A strong correlation was found between detector response of percentage dose difference and MLC positional error introduced (N = 350 introduced errors) for both HBB and FSD/BSD with coefficient of determination of 0.999 and 0.977, respectively. The relationship between detector response to MLC positional change was found to be 20.65%/mm for HBB and 11.14%/mm for FSD and BSD. At baseline, the mean MLC positional accuracy averaged across all leaves was 0.06 ± 0.27 mm (HBB), 0.04 ± 0.52 mm (FSD), −0.06 ± 0.51 mm (BSD). The mean MLC positional accuracy relative to baseline over the 6‐month assessment was found to be highly reproducible at 0.00 ± 0.12 mm (HBB; N = 28 weeks), −0.02 ± 0.19 mm (FSD; N = 6 months), −0.03 ± 0.19 mm (BSD; N = 6 months). Conclusions Positional accuracy of a novel jawless, double‐focused, double‐stacked MLC has been characterized and monitored over 6 months with an efficient, highly sensitive, and robust method using an ICP array. This routine QA method supplements calculation‐based IMRT QA for an online adaptive radio...

Characterization and longitudinal assessment of daily quality assurance for an MR‐guided radiotherapy (MRgRT) linac

“…Our MLC positional accuracy is in agreeance to our institutional weekly QA MLC positional reproducibility . Small MLC gravitational‐induced effects combined with isocenter dependency with gantry rotation are observed, and are amplified due to the high sensitivity of the small‐field (i.e., 0.40 cm 2 x 0.83 cm 2 ) daily QA procedure.…”

“…MLC positional accuracy at central axis and off axis was previously reported at our institution by Mittauer et al . using an ionization profiler and found to be within 0.06 ± 0.16 mm at baseline with no drift 0.00 ± 0.12 mm (n = 6 months) quantified using half beam block at cardinal angles about the central axis …”

“…Small MLC gravitational‐induced effects combined with isocenter dependency with gantry rotation are observed, and are amplified due to the high sensitivity of the small‐field (i.e., 0.40 cm 2 x 0.83 cm 2 ) daily QA procedure. Our weekly MLC QA is performed for each MLC bank evaluated independently. A similar technique could be employed for the daily QA in which the four cardinal angles are measured twice for each upper and lower MLC bank, (i.e., MLC1 and MLC2) to allow for bank differences to be uniquely determined.…”

“…The linac produces a 6‐MV flattening filter free beam with a nominal dose rate of 600 MU/min. Beam collimation is achieved using the RayZR™ MLCs, consisting of a set of two banks of MLCs, stacked and double focused, and offset by one‐half leaf width, eliminating the need for MLC tongue and groove design . With a 90 cm SAD, the MLCs project to a maximum field size of 27.4 cm 2 x 24.07 cm 2 at isocenter with individual leaf width projections of 8.3 mm.…”

Scintillation imaging as a high‐resolution, remote, versatile 2D detection system for MR‐linac quality assurance

“…However, for multileaf collimator (MLC) position verification, techniques have demonstrated high spatial sensitivity using an ion chamber array. 8 Plastic scintillators have been shown to be less affected than ion chambers by the presence of a transverse magnetic field 9 and therefore may be a strong candidate for MR-linac dosimetry. 1 Radiochromic film provides an MR-compatible high-resolution solution for many routine QA checks, however films are one-time use, must be replaced for different tests, and require post exposure readout procedures.…”