A novel unidirectional intensity map segmentation method for step-and-shoot IMRT delivery with segment shape control

In intensity modulated radiation therapy (IMRT), intensity maps are computed from prescribed doses to target volumes, adding dose restrictions to the surrounding tissues. Those intensity (fluence) maps are discretized into matrices of natural numbers and translated to sequences of multileaf collimator (MLC) leaf movements, which will finally deliver the computed x-ray intensities. A unidirectional leaf sequencing algorithm that controls the shape of the segments and reduces leaf motion time for step-and-shoot dose delivery is presented. The problem of constructing segments in controlling its shape was solved by synchronizing right leaves motion. This is done without increasing the number of segments, or the total number of monitor units, and taking into account the unidirectional leaf motion and the interleaf collision constraints. The method was tested using random matrices and a clinical case planned with the PCRT 3D® planning system. Compared to other unidirectional leaf sequencing methods, the proposed algorithm performs very similarly. But, in addition, the segment shape control produces segments with smoother outlines and more compact shapes, which may help to reduce MLC-specific effects when delivering the planned fluence map. Finally, as a result of imposing unidirectionality, this algorithm can be extended for sliding window segment generation.