A dosimetric comparison of two multileaf collimator designs

Abstract

We present the results of measurements designed to compare two different multileaf collimator (MLC) designs using a novel evaluation technique. The MLC designs evaluated were: a "single-focused" MLC (SF-MLC) mounted below the jaws, and a "double-focused" MLC, which is a complete replacement for the lower jaws. The ability of each MLC to conform isodose lines to a prescribed field edge (PFE) was evaluated using film dosimetry. Circular fields, centered on axis and off axis, were used because they produce a range of "angles of approach" between the MLC leaves and the PFE. They also have the advantage that for an ideal field shaping system the resulting isodoses are concentric perfect circles, a well-defined basis for evaluation. The amplitude of the oscillations of the 50% isodose line about the PFE and the penumbra width as determined by the 20%, 80%, and 90% isodose lines was examined. We observe that the 50% isodose line oscillates around the PFE with greater amplitude for SF-MLC. We attribute this, at least in part, to the rounded ends of the SF-MLC leaves. However, the SF-MLC has a noticeably sharper penumbra, which we attribute to its position further from the source. We conclude that these results are relevant for accurate dosimetric modeling of these devices.