Simultaneous minimization of segments and junctions in virtual micro IMRT

Virtual Micro IMRT combines a 10×5 mm² intensity map with a 5×10 mm² intensity map (delivered at orthogonal collimator settings). This yields a 5×5 mm² virtual micro intensity map (VMIM) that can be delivered with a 1 cm leaf width MLC. For each group of four 5×5 mm² cells (that can be covered by an MLC leaf in either collimator orientation), the minimum intensity can be delivered from either collimator setting. By varying the proportions of the minimum values that go into each component map, several pairs of maps can be generated and evaluated in terms of the number of segments and junctions. (These junctions occur when the gradients across cells in the component maps are in a direction opposite to the gradients at the same locations in the VMIM.) The optimization process looks for areas in the component maps where the gradients change sign and it reduces the difference in the gradients. Priority is given to arrays (rows or columns) with a large sum of positive gradients along the leaf motion direction. Ties are broken by choosing arrays with the fewest peaks. Within these arrays, priority is given to locations that experience a decrease in the number of junctions when the gradient difference is reduced. Compared to the result for a pair of maps that share the minimum value equally, the result of the optimization has 45% less junctions and 12% less segments for a 14×21 cm² VMIM. Siemens supported this research