Room: ePoster Forums
Purpose: A major cause of the difference between the final dose and optimization dose comes from the leaf sequencing stage in intensity modulated radiotherapy (IMRT). Traditional leaf sequencing methods use ideal point source to convert optimized fluence map to MLC segments without considering photon scatters inside LINAC head. In this study we incorporate an optimization step to optimize the leaf position for a better consistency between the optimized fluence and MLC generated fluence .
Methods: First, we formulated a cost function to measure the squared difference between the optimized fluence and MLC generated fluence which is a function of leaf positions taking radiation source scattering and flatten filter into account. A two-source model consisting of primary source and extra-focal source is used to calculate the fluence map given a set of MLC segments. Since the fluence map calculation process comprises of two convolution operations, the derivatives of the cost function of the MLC positions can be calculated. Finally, we use a gradient based optimization method to obtain the optimized MLC segments in an iterative manner.
Results: The proposed method was compared with a traditional static IMRT leaf sequencing algorithm and direct aperture optimization (DAO). We observe a better agreement between the final dose and optimization dose for our method. Since the proposed method needs no dose calculation in the optimization process, it has better computing performance compared with DAO which has to invoke dose calculation for each iteration.
Conclusion: A improved leaf sequencing method is proposed for implementation of intensity modulated beams. It has shown promising results with respect to the consistency between the optimized and MLC generated fluence map.