Room: Stars at Night Ballroom 1
Purpose: With advance MR-guided online adaptive radiotherapy relying on calculation-based IMRT QA, accurate and sensitive MLC QA is essential. We have developed and implemented an efficient and highly sensitive routine QA procedure using an ionization chamber profiler(ICP) array for real-time characterization of the positional accuracy of a double-focused and double-stack MLC on a clinical MR-guided radiotherapy(MRgRT) system.
Methods: An in-house MR-compatible jig positioned the ICP (resolution 5mm) at an extended SDD of 108.4cm to enable each MLC leaf(8.3mm leaf width at isocenter) to be uniquely determined by two-adjacent ion chambers. The MRgRT linac system utilizes a novel jawless, tongue-and-grooveless, double-focused, and double-stack MLC design such that the upper and lower bank are offset by half-a-leaf width. Positional accuracy was characterized by a half-beam block(HBB), forward-slash diagonal(FSD), backslash diagonal(BSD). The sensitivitity was evaluated by introducing 0.5-2.5mm known errors. The relationship between detector response and MLC error was established. We have evaluated MLC performance over a six-month longitudinal assessment.
Results: Correlation was found between detector response and MLC positional error (N=350 introduced errors) for HBB(rÂ²=0.999) and FSD/BSD(rÂ²=.977). The relationship between detector response (% dose difference) to MLC positional change (error introduced) was 20.65%/mm (HBB) and 11.14%/mm (FSD/BSD). At baseline mean positional accuracy was 0.06Â±0.16mm(HBB), 0.05Â±0.6mm(FSD), -0.05Â±0.7mm(BSD). The mean positional accuracy relative to baseline over six months was highly reproducible at 0.00Â±0.12mm(HBB), -0.02Â±0.19mm(FSD), -0.03Â±0.19(BSD). Larger positional errors(>1 mm) were observed at off-axis locations in both baseline and longitudinal assessment for FSD and BSD; attributed to the calibration being slightly defocused, where greater impact is observed at off-axis.
Conclusion: Positional accuracy of a novel jawless, double-focused, double-stack MLC has been characterized and monitored over six months with an efficient, highly sensitive and robust method. Longitudinal assessment demonstrated no-drift in the MLC calibration and positional accuracy to be within TG142 criteria for >95% of measurements.
Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees and consulting fees from ViewRay Inc. during conduct of this study. Dr. Bayouth reports membership on scientific advisory board of ViewRay Inc. during conduct of this study.