A novel method for treatment plan optimization

Physical or radiobiological objective functions are usually minimized either by determining the most appropriate beam filters or by optimizing individual beam geometries. The former case requires the use of some form of IMRT delivery system, while the latter technique may be employed in more conventional systems and where, perhaps, IMRT is inappropriate. Two current approaches to geometrical optimization involve minimizing physical beam overlap either with critical structures or with one another. Many standard treatment-planning techniques (e.g., wedged-pair plans), however, would not be obtained with either of these two approaches. Here, the authors present first results from a new method for static beam optimization. In this approach, an initial calculation is made for a large number of open conformal beams that are uniformly placed on a single isocenter located within the center of the PTV. Treatment geometries are limited only to the physical limits of the treatment setup. Using prototype software for the authors´ treatment-planning system, beam weights are optimized to minimize physical objective functions to determine those geometries most appropriate for the desired objectives. Eliminating those geometries with zero or negligible weighting creates a reduced set of beam geometries. A second calculation is then made with 4 copies of each remaining beam, adding 60° wedge filters in 4 orthogonal orientations. A second optimization is performed to obtain the desired beam arrangement, including wedge angle and orientation. Initial results for optimization of prostate treatment demonstrate sharp improvement over previously reported treatment techniques. Quantitative comparisons based on DVHs are presented