Use of a multileaf collimator to increase the field width achievable with a dynamic wedge

Abstract

A method is proposed for generating virtual wedges spanning the entire field width, defined as the collimator opening in the wedged direction, without changes to existent hardware. The technique approximates the fluence pattern of a dynamic wedge by sequentially closing the leaves of a 120 leaf MLC. Closure times for the individual leaves were derived by extending the segmented treatment table of the dynamic wedge provided by the manufacturer of the linear accelerator. Employing film dosimetry, beam properties of MLC-wedges were compared to those of conventional dynamic and mechanical wedges. Profiles and isodose lines of the MLC-wedge were almost identical to those of the dynamic wedge, and differed only modestly from the mechanical counterparts. Dose inhomogeneity due to the individually closing leaves was not significant. The high dose region at the junction between opposing MLC leaves, unavoidable when the field length (i.e., the opening of the collimator in the non-wedged direction) exceed the maximum leaf extension of 15 cm, was feathered by moving leaf pairs after their closure for the remainder of the irradiation time. Combining the MLC-wedge with a regular dynamic wedge to reduce the line of high dose under the leaf junction is under consideration.