Biological dose optimization with multiple ion fields.

We describe a method to irradiate arbitrarily shaped target volumes with simultaneously optimized multiple fields of fast carbon ions, explicitly taking into account sparing of organs at risk. The method was developed with realistic technical boundary conditions in mind, so that irradiations can be executed with devices like the GSI raster scanner or its successors at the upcoming dedicated ion-beam radiotherapy facilities. By virtue of the local effect model (LEM) biological effects are fully taken into account. Several minimization algorithms were investigated, and plain gradient search was found to be more effective than methods based on conjugate gradients or Newton's root finding algorithm. Two sets of cell survival experiments for the experimental verification of patient-like treatment plans were performed. Chinese hamster cells were used for quasi two-dimensional biological dosimetry. The plans combine a very good target conformation with an excellent sparing of organs-at-risk which was verified by the measurements. The results are compared to predictions of the local effect model in its original formulation and a modified version taking additional effects of clustered DNA damage into account. The new method is implemented in GSI's TRiP98 treatment planning system. It has already been applied clinically for planning and irradiating selected patients within the GSI pilot project.