A method to improve spatial resolution and smoothness of intensity profiles in IMRT treatment planning

In IMRT optimization, the size of beamlets used for optimizing the beam intensity distributions is a planner-selected parameter. The appropriate setting for the beamlet size is critical to the outcome of IMRT planning. With too small beamlets the dose calculation can be inaccurate, and the resulting intensity profiles can be unnecessarily complex and difficult to generate. Relatively simple intensity profiles can be obtained using large beamlets. However, this may compromise the conformity of the dose distribution. In this paper we present a method, in which multiple beamlet matrices displaced from each other by a shift in MLC leaf travel direction are used instead of the single beamlet matrix per beam in a conventional method, to achieve finer spatial resolution for the intensity distribution than the given beamlet size. Two test cases were used to assess the method by the resultant DVHs and dose distributions and characteristic indices of the intensity profiles. The results show that this method can produce optimized dose distributions that are similar to those produced by the conventional inverse planning method with the benefit of smoother intensity profiles that are easier to deliver with a computer controlled MLC.