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A Multi-Institutional Evaluation of Multileaf Collimator Performance

J Kerns

J Kerns1*, N Childress2, S Kry3, (1) MD Anderson Cancer Ctr., Houston, TX, (2) Mobius Medical Systems, LP, HOUSTON, TX, (3) MD Anderson Cancer Ctr., Houston, TX

TU-E-108-7 Tuesday 2:00PM - 3:50PM Room: 108

Purpose: This study examined MLC positional accuracy via MLC logs from multiple institutions and multiple delivery techniques to evaluate typical positional accuracy and treatment and mechanical parameters that affect accuracy. Typical accuracy achieved was compared against TG-142 recommendations for MLC performance; more appropriate recommendations are suggested.

Methods: Over 85,000 Varian MLC treatment logs were collected from six institutions and analyzed with FractionCHECK. Data were binned according to institution and treatment type to determine root mean square (RMS) and 95th percentile error values, and then to look for correlations between those errors and with mechanical and treatment parameters.

Results: Results of treatment logs from six institutions found that leaf RMS error and 95th percentile leaf error were fairly consistent between institutions, but varied by treatment type. The step and shoot technique had very small errors: the mean RMS leaf error was 0.008 mm. For dynamic treatments the mean RMS leaf error was 0.32 mm, while VMAT showed the largest mean RMS leaf error at 0.46 mm. For the dynamic and VMAT techniques, the mean and maximum leaf speeds were significantly linked to the leaf RMS error. For dynamic delivery, the mean leaf error was correlated with RMS error, whereas for VMAT the average gantry speed was correlated. For all treatments, the RMS error and the 95th percentile leaf error were correlated.

Conclusion: Restricting the maximum leaf speed can help improve MLC performance for dynamic and VMAT deliveries. Furthermore, the tolerances of leaf RMS and error counts for all treatment types should be tightened from the TG-142 values to make them more appropriate for clinical performance. Values of 1 mm for the 95th percentile of leaf RMS error and 1.5 mm for the 95th percentile error are suggested as action levels for all treatment types.

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