Intensity modulated radiotherapy with charged particle beams: studies of inverse treatment planning for rotation therapy.

The optimization of intensity modulated radiotherapy (IMRT) for charged particle beams is a necessary prerequisite to evaluate the clinical potential of this treatment modality in comparison to IMRT with high energy photons. A theoretical study for IMRT with charged particle beams delivered by rotation therapy is presented. First, the inverse problem for two-dimensional rotation therapy with arbitrary depth dose curves is formulated. Then a numerical strategy is devised to calculate fluence profiles for the simplified case of arbitrary rotationally invariant dose distributions. This mathematical framework is applied to study various aspects of charged particle IMRT. A central topic of the investigation is the evaluation of dose delivery, based on distal edge tracking (DET) and intensity modulation. The potential of DET-IMRT with charged particle beams is studied in comparison to an optimal, conventional dose delivery technique, which employs the concept of a spread-out Bragg peak (SOBP). Moreover, a comparison to photon IMRT is provided for simple geometric dose patterns. The technique of DET-IMRT for the delivery of a homogeneous target dose is only feasible for targets up to a critical radius, depending on the individual shape of the employed Bragg peak. The irradiation of larger targets requires energy modulation in addition to the range modulation for DET. The accurate placement of the Bragg peak with respect to the target edge is found to be of potential importance. Comparing dose delivery via DET-IMRT with the optimal SOBP technique revealed a significant advantage of DET-IMRT, especially a saved integral dose in target-adjacent healthy tissues of up to 30%, and a reduction of the penumbra at the target edge by almost 50%. A saving in integral dose to healthy tissues by a factor of 2-3 was observed for DET-IMRT in comparison to IMRT with high energy photons.