A multi-institution evaluation of MLC log files and performance in IMRT delivery

Abstract

Background: The multileaf collimator (MLC) is a critical component to accurate intensity-modulated radiotherapy (IMRT) delivery. This study examined MLC positional accuracy via MLC logs from Varian machines from six institutions and three 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 overall root mean square (RMS) and 95th percentile error values, and then to look for correlations between those errors and with mechanical and treatment parameters including mean and maximum leaf speed, gantry angle, beam-on time, mean leaf error, and number of segments.

Results: Results of treatment logs found that leaf RMS error and 95th percentile leaf error were 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 volumetric-modulated arc treatment (VMAT) showed an RMS leaf error of 0.46 mm. Most MLC leaf errors were found to be well below TG-142 recommended tolerances. For the dynamic and VMAT techniques, the mean and maximum leaf speeds were significantly linked to the leaf RMS error. Additionally, 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.

Conclusions: 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 leaf error are suggested as action levels for all treatment types.

Figure 1

Box plot of individual leaf RMS error values and 95 ^th percentile values by treatment type. (A-C) Leaf RMS error results plotted by institution for step & shoot, dynamic, and VMAT modalities respectively. (D-F) 95^th percentile errors plotted by institution for step & shoot, dynamic and VMAT modalities respectively. Results are plotted as normal boxplots with the top and bottom of the rectangle representing the 75^th (q3) and 25^th (q1) percentile values respectively and the median as the line within the box. The upper whisker corresponds to q3 + 1.5*(q3 – q1) and lower whisker to q1 – 1.5*(q3 – q1) or to the nearest data point thereof. Values above or below the respective whiskers are plotted as outliers.

Figure 2

Aggregated step and shoot treatment data plotting treatment parameters. Individual leaf or bank RMS error is plotted according to (A) gantry angle, (B) beam-on time, (C) number of segments, (D) 95^th percentile error, (E) mean leaf error with each bank plotted separately. For (A-D), data density is shown by color change; blue has low density while red has the highest density.

Figure 3

Aggregated dynamic treatment data plotting treatment parameters. Individual leaf or bank RMS error is plotted according to (A) mean leaf speed, (B) maximum leaf speed, (C) gantry angle, (D) beam-on time, (E) number of segments, (F) number of beam holdoffs, (G) 95^th percentile error, and (H) mean leaf error with each bank plotted separately. For (A-G), data density is shown by color change; blue has little density while red has the highest density.

Figure 4

Aggregated VMAT data plotting treatment parameters. Individual leaf or bank RMS error are plotted according to (A) mean leaf speed, (B) maximum leaf speed, (C) average gantry speed throughout the treatment, (D) beam-on time, (E) 95^th percentile error, and (F) mean leaf error with each bank plotted separately. For (A-E), data density is shown by color change; blue has low density while red has the highest density.

Figure 5

In-depth analysis of VMAT logs. Snapshots of 1 second intervals for every VMAT treatment log are plotted. Each snapshot/data point is the speed of the fastest leaf and the gantry speed.