Treatment planning of intensity modulated composite particle therapy with dose and linear energy transfer optimization

The biological effect of charged-particle beams depends on both dose and particle spectrum. As one of the physical quantities describing the particle spectrum of charged-particle beams, we considered the linear energy transfer (LET) throughout this study. We investigated a new therapeutic technique using two or more ion species in one treatment session, which we call an intensity modulated composite particle therapy (IMPACT), for optimizing the physical dose and dose-averaged LET distributions in a patient as its proof of principle. Protons and helium, carbon, and oxygen ions were considered as ion species for IMPACT. For three cubic targets of 4 x 4 x 4, 8 x 8 x 8, and 12 x 12 x 12 cm(3), defined at the center of the water phantom of 20 x 20 x 20 cm(3), we made IMPACT plans of two composite fields with opposing and orthogonal geometries. The prescribed dose to the target was fixed at 1 Gy, while the prescribed LET to the target was varied from 1 keV mu m(-1) to 120 keV mu m(-1) to investigate the range of LET valid for prescription. The minimum and maximum prescribed LETs, (L-T_ (min), L-T_ (max)), by the opposing-field geometry, were (3 keV mu m(-1), 115 keV mu m(-1)), (2 keV mu m(-1), 84 keV mu m(-1)), and (2 keV mu m(-1), 66 keV mu m(-1)), while those by the orthogonal-field geometry were (8 keV mu m(-1), 98 keV mu m(-1)), (7 keV mu m(-1), 72 keV mu m(-1)), and (8 keV mu m(-1), 57 keV mu m(-1)) for the three targets, respectively. To show the proof of principle of IMPACT in a clinical situation, we made IMPACT plans for a prostate case. In accordance with the prescriptions, the LETs in prostate, planning target volume (PTV), and rectum could be adjusted at 80 keV mu m(-1), at 50 keV mu m(-1), and below 30 keV mu m(-1), respectively, while keeping the dose to the PTV at 2 Gy uniformly. IMPACT enables the optimization of the dose and the LET distributions in a patient, which will maximize the potential of charged-particle therapy by expanding the therapeutic window. Further studies and developments will enable this therapeutic technique to be used in clinical practice.