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

The optimization of intensity modulated radiotherapy (IMRT) for charged par ticle 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 rotat ion therapy is presented. First, the inverse problem for two-dimensional ro tation therapy with arbitrary depth dose curves is formulated. Then a numer ical strategy is devised to calculate fluence profiles for the simplified c ase of arbitrary rotationally invariant dose distributions. This mathematic al framework is applied to study various aspects of charged particle IMRT. A central topic of the investigation is the evaluation of dose delivery, ba sed on distal edge tracking (DET) and intensity modulation. The potential o f DET-IMRT with charged particle beams is studied in comparison to an optim al, conventional dose delivery technique, which employs the concept of a sp read-out Bragg peak (SOBP). Moreover, a comparison to photon IMRT is provid ed for simple geometric dose patterns. The technique of DET-IMRT for the de livery of a homogeneous target dose is only feasible for targets up to a cr itical 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 w ith respect to the target edge is found to be of potential importance. Comp aring dose delivery via DET-IMRT with the optimal SOBP technique revealed a significant advantage of DET-IMRT, especially a saved integral dose in tar get-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 tiss ues by a factor of 2-3 was observed for DET-IMRT in comparison to IMRT with high energy photons. (C) 2000 American Association of Physicists in Medici ne. [S0094-2405(00)00506-X].