Spatial dependence of MLC transmission in IMRT delivery.

Phys Med Biol 2007 Oct 17;52(19):5985-99. Epub 2007 Sep 17.

Department of Radiation Oncology, Mannheim Medical Centre, University of Heidelberg, 68167 Mannheim, Germany.

In complex intensity-modulated radiation therapy cases, a considerable amount of the total dose may be delivered through closed leaves. In such cases an accurate knowledge of spatial characteristics of multileaf collimator (MLC) transmission is crucial, especially for the treatment of large targets with split fields. Measurements with an ionization chamber, radiographic films (EDR2, EBT) and EPID are taken to characterize all relevant effects related to MLC transmission for various field sizes and depths. Here we present a phenomenological model to describe MLC transmission, whereby the main focus is the off-axis decrease of transmission for symmetric and asymmetric fields as well as on effects due to the tongue and groove design of the leaves, such as interleaf transmission and the tongue and groove effect. Data obtained with the four different methods are presented, and the utility of each measurement method to determine the necessary model parameters is discussed. With the developed model, it is possible to predict the relevant MLC effects at any point in the phantom for arbitrary jaw settings and depths.

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http://stacks.iop.org/0031-9155/52/i=19/a=018?key=crossref.1
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http://dx.doi.org/10.1088/0031-9155/52/19/018DOI Listing
October 2007
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