Multileaf Collimator Quality Assurance Research Articles

SU‐FF‐T‐161: Practical Gap‐Width Threshold for MLC Quality Assurance for IMRT

Purpose: A standard IMRT quality assurance test for Multi Leaf Collimator (MLC) leaf position accuracy is the Memorial Sloan‐Kettering (MSK) leaf test. This test involves inspecting the widths of five 1 mm wide fields delivered 2 cm apart. A maximum error tolerance of 0.2 mm in the gap‐width between sets of opposing leaves is recommended. In a preliminary informal visual test performed with an independent observer, the observer detected all deliberately introduced errors greater than or equal to 0.5 mm, but not all 0.2 mm errors. Method and Materials: We investigated the use of the MSK leaf test on a Varian Clinac 2300C/D and Clinac 2300EX, with Mark I (52 leaf) and Millennium (120 leaf) MLCs, respectively. Kodak XV film was irradiated with 6 MV photons at an SSD of 100 cm, using 1000 monitor units for the delivery of the five segments. The film was scanned using a Vidar VXR‐16DP scanner, and subsequently analyzed with the RIT113 Version4 software. The RIT software recommends using a 3×3 median filter to remove background noise, which effectively reduces the image resolution. Since the smallest baseline resolution of the scanner is 0.089 mm, a 0.2mm error will be difficult to observe after a 3×3 filtration. Separate films were irradiated under the same conditions and scanned to determine if the scanner software combination was consistent in assessment of the magnitude of errors in the gap‐width. Results: The reproducibility of leaf positions on multiple films was found to be within one pixel. However, not all artificially introduced errors that were visually observable were detected by this software technique. Conclusion: It may not be feasible to reliably measure gap‐width errors of 0.2 mm, either by eye or by quantitative analysis using the technique presented here.

MLC quality assurance techniques for IMRT applications.

Intensity modulated radiotherapy (IMRT) requires extensive knowledge of multileaf collimator (MLC) leaf positioning accuracy, precision, and long-term reproducibility. We have developed a technique to efficiently measure the absolute position of each MLC leaf, over the range of leaf positions utilized in IMRT, based on dosimetric information. A single radiographic film was exposed to 6 MV x-rays for twelve exposures: one open field with a radio-opaque marker tray present, and eleven fields (1 x 28 cm strips via 1 cm gaps between opposed leaf pairs) separated by 2 cm center to center. The process was repeated while varying direction of leaf travel; each film was digitized using a commercial film dosimetry system. The digital images were manipulated to remove translation and rotation of the film data with respect to the collimator coordinate system by extraction of radiation dose profiles perpendicular to the MLC leaf motion and measuring the center of the x-ray leakage between leaves. Radiation dose profiles in the direction of leaf motion were acquired through the center of each leaf pair (leaves 2-28), which provided leaf position information every 2 cm with 0.2 mm precision. Nine separate leaf reproducibility studies over a 90 day period which evaluated 600 measurement points on each film show 0.3 mm precision for 95% confidence, while hysteresis studies show 0.5 mm precision. Absolute leaf position error measurements demonstrated a radial dependence, with a maximum of 1.5 mm at 16.4 cm from central axis, due to rotational error at calibration. Recalibration of the MLC leaves based utilizing this tool yields absolute leaf position measurements where 91.5% of all leaves/positions were within 0.5 mm, with a mean error of 0.1 mm and a maximum error less than 1.0 mm.