Selection of beam angles for radiotherapy of skull base tumours using charged particles.

The geometrical treatment configuration for skull base tumours is investigated using three different sources of information. First, an analysis of treatment angle combinations for 50 patients treated with conventional conformal radiation therapy is performed. In a second step, nine treatment plans for a hypothetical heavy ion gantry were prepared. Finally, cylindrical projections of the organs at risk are introduced as a tool to analyse the distribution of treatment angles as well as the treatment geometry for each patient. The results of the analysis for conventional therapy clearly show treatment angle combinations that are preferably used. These findings are also supported by the geometrical analysis using the cylindrical projections. The majority of treatment angles for heavy ions also seem to be confined to those regions. In one case, however, the sharp distal dose fall-off of the heavy ions allowed a beam direction outside these conventionally used regions. The advantages of a heavy ion gantry versus a fixed horizontal beam line are shown by comparison of treatment plans for both systems. Treatment angle combinations are suggested that should be accessible for any beam line design using either fixed beams or a gantry.