Verification of multileaf collimator leaf positions using an electronic portal imaging device

Abstract

An automated method is presented for determining individual leaf positions of the Siemens dual focus multileaf collimator (MLC) using the Siemens BEAMVIEW(PLUS) electronic portal imaging device (EPID). Leaf positions are computed with an error of 0.6 mm at one standard deviation (sigma) using separate computations of pixel dimensions, image distortion, and radiation center. The pixel dimensions are calculated by superimposing the film image of a graticule with the corresponding EPID image. A spatial correction is used to compensate for the optical distortions of the EPID, reducing the mean distortion from 3.5 pixels (uncorrected) per localized x-ray marker to 2 pixels (1 mm) for a rigid rotation and 1 pixel for a third degree polynomial warp. A correction for a nonuniform dosimetric response across the field of view of the EPID images is not necessary due to the sharp intensity gradients across leaf edges. The radiation center, calculated from the average of the geometric centers of a square field at 0 degrees and 180 degrees collimator angles, is independent of graticule placement error. Its measured location on the EPID image was stable to within 1 pixel based on 3 weeks of repeated extensions/retractions of the EPID. The MLC leaf positions determined from the EPID images agreed to within a pixel of the corresponding values measured using film and ionization chamber. Several edge detection algorithms were tested: contour, Sobel, Roberts, Prewitt, Laplace, morphological, and Canny. These agreed with each other to within < or = 1.2 pixels for the in-air EPID images. Using a test pattern, individual MLC leaves were found to be typically within 1 mm of the corresponding record-and-verify values, with a maximum difference of 1.8 mm, and standard deviations of <0.3 mm in the daily reproducibility. This method presents a fast, automatic, and accurate alternative to using film or a light field for the verification and calibration of the MLC.

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

“…One technique by Samant et al utilized several edge detection algorithms to track the leaf trajectory. This approach requires a fast sampling rate for sliding window IMRT techniques . The other approach is to directly use integral dose or fluence maps.…”

Task Group 142 report: Quality assurance of medical acceleratorsa)

“…Recent development of tools utilizing EPID devices allow for subpixel precision to detect changes in discrete locations of an acquired image. 69,70 As treatment planning parametrization seeks a global value for leaf transmission, it is important that the leaf body, side, and end characteristics do not change over time, the most vulnerable being the leaf side rigidity due to leaf inderdigitation, as it may affect interleaf leakage, hence the close attention needed. Leaf position repeatability, MLC spoke shot, and coincidence of light field and x-ray field all are tests intended to check the alignment of the MLCs.…”

A multileaf collimator phantom for the quality assurance of radiation therapy planning systems and CT simulators

“…Previous phantoms have proven to be useful tools for the QA of dosimetric parameters ((3)), and other aspects of dosimetry (penumbra, leaf transmission) related to the use of MLCs and the QA of the MLC component itself have been investigated (12)(13)(14). It is essential, however, to have tools for the QA of all aspects of the treatment planning process (2), and this phantom's design allows for the nondosimetric aspects to be addressed with respect to treatment planning systems.…”