Optimization by simulated annealing of three-dimensional, conformal treatment planning for radiation fields defined by a multileaf collimator: II. Inclusion of two-dimensional modulation of the x-ray intensity.

Interest is rapidly growing in using multiple x-radiation fields defined by a multileaf collimator to achieve conformal radiotherapy. Three-dimensional treatment planning in such situations is in its infancy and most 3D planning systems provide no tools for optimizing therapy. A previous paper addressed how to calculate optimum beamweights when both the target volume and all or some parts of organs at risk were in the fields-of-view. This work allowed a maximum of two weights per field. The present paper extends this technique to allow each radiation port to be spatially modulated across the geometrically shaped field. An optimization method based on simulated annealing is presented. It is shown that including spatial modulation leads to a wider separation between the dose-volume histograms of the target volume and organs at risk. The improvement is quantified in terms of the tumour control probability at constant normal tissue complication probability. Possible limitations of the a posteriori applied biological model are discussed in detail.