RBE intercomparison of proton beams at five radiotherapy facilities in Japan

Full text: Five proton therapy facilities (HIBMC, Hyogo; NCCHE, Kashiwa; WERC, Tsuruga; PMRC, Tsukuba; SCC, Shizuoka) are now working in Japan. Prior to starting the clinical use of proton-beams for cancer therapy, we have investigated the relative biological effectiveness (RBE) to high-energy X-rays. The RBE values were determined to estimate the clinical doses for each therapeutic facility. Spread-out Bragg peak (SOBP) beams having 60 mm depth were obtained from 180 MeV (WERC), 200 MeV (PMRC) or 190 MeV (the others) proton beams with the same way to the patient therapy at each facility. Human salivary gland tumor cell (HSG) that has used at the pre-clinical radiobiological studies (RBE determination) for HIMAC carbon beams was used. We obtained D10 values by means of in vitro colony formation assay after single-dose exposures of proton-beams. The RBE values of each beam were obtained by comparing the D10 values to that for 4-6 MeV linac X-rays at each facility. The D 10 values for high energy X-rays showed no significant differences among the facilities tested. The average RBE value for proton beams at the middle position of the SOBP was 1.03 ±±0.02 (mean ±SD). The values may be slightly lower at the entrance position of the SOBP, and a little higher at the distal position of the SOBP than that at middle position for WERC facility. The depth-dose distribution of all the SOBP beams has designed and obtained very well, and showed good uniformities of biological dose distribution for all facilities