On the use of the MLC dosimetric leaf gap as a quality control tool for accurate dynamic IMRT delivery

Purpose: MLC leaf gap consistency is critical for the accurate delivery of dynamic IMRT plans. It is estimated that a systematic MLC leaf gap change of 0.6 mm will result in a 2% change to the equivalent uniform dose to a clinical target volume for a typical head and neck sliding window (SW) IMRT plan. The aim of this work is to use the measured dosimetric leaf gap (DLG) to verify the dosimetric reproducibility of dynamically delivered SW IMRT plans. This study focuses on Varian linacs equipped with the 120 Millennium MLC and the Eclipse T treatment planning system (TPS), but can be extended to other linac/MLC/TPS combination. Methods: An ionization chamber, a diode array, and an electronic portal imaging device (EPID) were used to assess the DLG in zero( central axis(, one, and two dimensions, respectively. The DLG for zero and two dimensions was derived from measurements of SW fields of decreasing width (2, 1.5, 1, and 0.5 cm). The DLG in one dimension was measured directly from a single SW sweeping across a linear diode array. This one- dimensional DLG measurement was based on the full width at half maximum( FWHM( of the dose rate versus time spectrum. Results: The DLG derived from ion chamber measurements at central axis agrees to within 0.1 mm, with the DLG measured directly from the FWHM of dose rate versus time spectrum. The measured DLG depends on the control points used for the MLC SW fields. When two control points were used, the DLG measured at central axis showed an increase of 0.6 mm with respect to the same measurements performed using three or more control points. The two- dimensional distribution of DLG obtained using the EPID identified leaf gap errors as small as +/- 0.2 mm in isolated areas away from central axis. Conclusions: Comprehensive measurements of the DLG in 0D, 1D, and 2D provide an accurate assessment of DLG value required during TPS commissioning. These DLG measurements can also be used as a quality control tool to quantify changes of the MLC calibration and leaf gap consistency, which is critical for the accurate delivery of dynamically delivered SW IMRT plans. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3567148]