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

稲庭, 拓  ,  兼松, 伸幸  ,  野田, 耕司  ,  鎌田, 正

The biological effect of charged-particle beams depends on radiation quality represented practically by the linear energy transfer (LET). Thus, the dose and the LET should be optimized for ideal charged-particle therapy. 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 dose and LET distributions in a patient. Protons and helium, carbon, and oxygen ions were considered as ion species for IMPACT. For three cubic targets of 4 × 4 × 4, 8 × 8 × 8, and 12 × 12 × 12 cm3 defined at the center of the water phantom of 20 × 20 × 20 cm3, we made IMPACT plans of two 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/μm to 120 keV/μm to investigate the adjustable range of LET. The minimum and maximum prescribed LETs, (LT_min, LT_max), by the opposing-field geometry were (3 keV/μm, 115 keV/μm), (2 keV/μm, 84 keV/μm), and (2 keV/μm, 66 keV/μm), while those by the orthogonal-field geometry were (8 keV/μm, 98 keV/μm), (7 keV/μm, 72 keV/μm), and (8 keV/μm, 57 keV/μm) for the three targets, respectively. To show the availability of the technique in a clinical situation, we made IMPACT plans for a prostate case. In accordance with the prescriptions, LETs in prostate, planning target volume (PTV), and rectum could be adjusted at 80 keV/μm, at 50 keV/μm, and below 30 keV/μm, 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.

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