Optimisation of MLC orientation to improve accuracy in the static field delivery of IMRT

The authors have previously investigated algorithms which optimise the accuracy of delivery of Intensity Modulated radiotherapy (IMRT) beams using dynamic Multileaf Collimator (MLC) techniques. They showed that a high degree of agreement between idealised profiles and those delivered could be obtained in the direction of leaf trajectory. This is achieved by optimising the number and resolution of MLC steps and using optimally incremented intensity levels in the leaf sequence. It is clear that the width of the MLC leaves place limitations on any optimisation, restricting the resolution obtainable in the direction perpendicular to leaf motion. The authors have implemented an algorithm which samples an IMRT (square) field and finds the best MLC orientation in an attempt to improve this situation. Initially, the MLC leaf trajectories are aligned parallel to one major axis of the square. The intensity profile considered is a continuous function describing a surface. It is sampled at one degree intervals, rotating the MLC about the centre of the field through a total of 90 degrees. For the general collimator angle, a set of (1-D) profiles to represent the required intensify surface under each leaf pair are computed, with a width equal to that of the MLC leaf. The profiles are selected by minimising a root-mean-square error function representing the difference between desired and produced intensity maps. The authors find that general (non-zero) collimator angles, thus MLC orientations, can minimise differences between the idealised (required) 2-D intensity surfaces and that produced by IMRT using dynamic MLC methods