Does the option to rotate the Elekta Beam Modulator MLC during VMAT IMRT delivery confer advantage?--a study of 'parked gaps'.

When delivering intensity-modulated radiation therapy (IMRT) using the volumetric modulated arc therapy (VMAT) technique on an Elekta accelerator equipped with the Elekta Beam Modulator multileaf collimator (MLC), the orientation of the MLC, relative to the accelerator head, is generally fixed during the delivery. However, it has the ability to rotate about its axis as the gantry simultaneously rotates. This note shows that this can confer a potential advantage when planning and delivering IMRT via VMAT. A computer model has been built in which the MLC rotation angle could be varied with each control point (gantry location) within the constraints of the specified MLC rotation speed and the time available for rotation. The model was used to optimize the orientation trajectory in such a way as to minimize the number of parked gaps between leaves which are needed for some gantry orientations but not for others (and which cannot reach the shielding safety of surrounding jaws in the time available). The presented work started with the simple situation of collimating gantry-successive single convex shapes. As a broad statement some 40% reduction in such parked gaps could be achieved. The study was then extended to investigate the optimized trajectories for multiple separate concave shapes with, once again, a saving in unwanted parked gaps or unwanted over-irradiation.