A novel 2D binary collimator for IMRT dose delivery: dosimetric characterization using Monte Carlo simulations

We present a novel technical concept of a two-dimensional binary multileaf collimator (2D-bMLC) especially designed for fast dose delivery in rotational IMRT. The 2D-bMLC consists of individually controlled absorber channels, which are arranged side by side forming a 2D collimator aperture. In each channel three separate tungsten modules are arranged behind each other. To open and close an element, the central module is shifted between two positions. The purpose of this work is the presentation of the 2D-bMLC concept and its dosimetric evaluation. To determine the dosimetric properties, we designed a Monte Carlo model of an exemplary 2D-bMLC, consisting of 30 × 30 elements. A virtual source model of a flattening filter-free 7 MV linac was used to characterize the linac phase space. A primary radiation efficiency factor of 43% was calculated for the open 2D-bMLC by dividing the integral dose scored for a 2D-bMLC field by the integral dose scored for an open field with the same dimensions. The leakage calculated for the closed collimator was below 0.5%. Following the primary photon fluence distribution, the bixel intensity decreases with the distance of the element to the central axis of the treatment machine. From the collimator field's center toward its borders, the geometric bixel widths increase in a symmetric and predictable manner by up to 4%. The increase is explained by the specific design of the 2D-bMLC. Abutting element beams exhibit a slight tongue-and-groove effect if opened sequentially. This effect as well as the primary radiation efficiency is basically affected by the source size and the dimensions of the collimator elements. We successfully established and evaluated a dosimetric model of the 2D-bMLC. The results are promising, and we will therefore investigate on real patient plans, if the concept could be advantageous for fast rotational IMRT treatments.