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

The geometrical treatment configuration for skull base tumours is investiga ted using three different sources of information. First, an analysis of tre atment angle combinations for 50 patients treated with conventional conform al radiation therapy is performed. In a second step, nine treatment plans f or a hypothetical heavy ion gantry were prepared. Finally, cylindrical proj ections of the organs at risk are introduced as a tool to analyse the distr ibution of treatment angles as well as the treatment geometry for each pati ent. The results of the analysis for conventional therapy clearly show trea tment angle combinations that are preferably used. These findings are also supported by the geometrical analysis using the cylindrical projections. Th e majority of treatment angles for heavy ions also seem to be confined to t hose regions. In one case, however, the sharp distal dose fall-off of the h eavy ions allowed a beam direction outside these conventionally used region s. The advantages of a heavy ion gantry versus a fixed horizontal beam line are shown by comparison of treatment plans for both systems. Treatment ang le combinations are suggested that should be accessible for any beam line d esign using either fixed beams or a gantry.