Antiprotons Comparison of optimized single and multifield irradiation plans of antiproton, proton and carbon ion beams Niels Bassler a,b, * , Ioannis Kantemiris c , Pantelis Karaiskos d , Julia Engelke b , Michael H. Holzscheiter e,f , Jørgen B. Petersen g a Dept. of Experimental Clinical Oncology, Aarhus University Hospital, Århus, Denmark; b Deutsches Krebsforschungszentrum, Heidelberg, Germany; c Nuclear and Particle Physics Section, Physics Department; and d Medical Physics Laboratory, Medical School, University of Athens, Athens, Greece; e Dept. of Physics & Astronomy, University of New Mexico, Albuquerque, USA; f Max Planck Institute für Kernphysik, Heidelberg, Germany; g Department of Medical Physics, Aarhus University Hospital, Århus, Denmark article info Article history: Received 31 August 2009 Received in revised form 17 February 2010 Accepted 23 February 2010 Available online 19 March 2010 Keywords: Antiproton therapy Proton therapy Carbon ion therapy Treatment planning Particle therapy abstract Background and purpose: Antiprotons have been suggested as a possibly superior modality for radiother- apy, due to the energy released when antiprotons annihilate, which enhances the Bragg peak and intro- duces a high-LET component to the dose. However, concerns are expressed about the inferior lateral dose distribution caused by the annihilation products. Methods: We use the Monte Carlo code FLUKA to generate depth–dose kernels for protons, antiprotons, and carbon ions. Using these we then build virtual treatment plans optimized according to ICRU recom- mendations for the different beam modalities, which then are recalculated with FLUKA. Dose–volume histograms generated from these plans can be used to compare the different irradiations. Results: The enhancement in physical and possibly biological dose from annihilating antiprotons can sig- nificantly lower the dose in the entrance channel; but only at the expense of a diffuse low dose back- ground from long-range secondary particles. Lateral dose distributions are improved using active beam delivery methods, instead of flat fields. Conclusions: Dose–volume histograms for different treatment scenarios show that antiprotons have the potential to reduce the volume of normal tissue receiving medium to high dose, however, in the low dose region antiprotons are inferior to both protons and carbon ions. This limits the potential usage to situa- tions where dose to normal tissue must be reduced as much as possible. Ó 2010 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 95 (2010) 87–93 Reducing unwanted radiation-induced effects in the normal tis- sue region is one of the main goals of radiotherapy and has moti- vated the research for new beam modalities and beam delivery techniques, such as IMRT [1] and charged particle therapy with protons and heavier ions [2–5]. Radiotherapy with antiproton beams has earlier been hypothesized to offer a potential therapeu- tic advantage due to the additional energy released at the annihi- lation vertex [6–8]. Primarily, antiproton annihilation takes place when the particle has slowed down and can be captured by a nu- cleus of the target material, when it will annihilate as described in [9]. The energy released hereby is close to 2 GeV, however, accord- ing to Sullivan et al. [7], only up to 30 MeV of the released energy is confined to a region near the annihilation vertex, doubling the local energy deposition compared to protons. The secondary particle flu- ence and energy spectrum is described in the Refs. [10,9,11]. The remainder of the total mass energy released during the annihila- tion event will be carried off by high and medium energy second- ary particles like pions (charged and neutral), muons, high energy photons, and neutrons. The energy deposited outside of the plan- ning target volume (PTV) by these secondary particles is of great concern and must be carefully studied in order to assess the ther- apeutic value of antiprotons. This has recently been pointed out by Paganetti et al., in this issue [11], who demonstrated that due to the large penumbra of the pristine antiproton beam, only very poor lateral dose distributions can be achieved using flat fields. However, we feel that the very simplistic fields used by Paga- netti et al. is far from a fair comparison of physical dose distribu- tions, which we here will demonstrate with a few simple optimizations. Even if our simple optimization approach presented here may be somewhat remote from clinical practice we still argue that our comparative approach is closer to clinical reality, contrary to the approach taken by Pagenetti et al. We simulate a simple water phantom where the planned target volume (PTV) is irradiated with protons, antiprotons and carbon ions, individually optimizing the treatment plans for the different particles to meet the conditions given for the PTV by the ICRU Re- port 78 [12], while at the same time minimizing the impact on any volume outside of the PTV. 0167-8140/$ - see front matter Ó 2010 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2010.02.026 * Corresponding author. Address: Dept. of Experimental Clinical Oncology, Aarhus University Hospital, Århus, Denmark. E-mail address: bassler@phys.au.dk (N. Bassler). Radiotherapy and Oncology 95 (2010) 87–93 Contents lists available at ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com