Biological dose optimization using ramp-like dose gradients in ion irradiation fields

We describe a method to improve treatment planning for carbon ion radiotherapy using biologically optimized inhomogeneous dose prescriptions of opposing fields. The biological optimization of single fields is performed in such a way that a linear rise or fall-off of the dose is produced over the target volume. The superposition of these inhomogeneous fields yields a homogeneous dose distribution with improved tissue sparing for concave target volumes. The variation of the relative biological effectiveness (sp(rbe) is fully taken into account. Schematic as well as real patient plants show significant improvements in the dose-volume histograms for ramp-like dose gradients across the target volume in beam's eye-view direction. The method is applicable to ions other than carbon as well.