A new concept of multileaf collimator (the shuttling MLC)--an interpreter for high-efficiency IMRT

This paper proposes a radically new concept for a multileaf collimator for a photon linear accelerator for delivering IMRT with high monitor-unit efficiency. The concept is to consider each M (rows) x N (columns) two-dimensional intensity-modulated beam (2D IMB) as a set of N/2 M (rows) x 2 (columns) areas of modulation. Each area is then delivered by a set of M shuttling attenuating elements (called here the shuttling MLC) with a very high monitor-unit efficiency. The elements shuttle between each of the two columns comprising the M x 2 area and the modulation is provided by the variation in dwell time of the elements. The principles of this shuttling multileaf collimator are discussed and examples illustrating its operation are given. The main achievement reported in this paper is the development and robust testing of an interpreter which describes the position-time course of movement of the elements as a function of monitor units. This interpreter fully accounts for leakage transmission through the elements. It completely avoids the across-the-rows tongue-and-groove underdose. A large number of ID and 2D IMBs have been subjected to this interpreter and it is shown that for random patterns of fluence the SMLC is more monitor-unit efficient than the Bortfeld-Boyer technique (the most efficient with a conventional MLC) when the modulation is highly structured.