Relative biological effectiveness of the 235 MeV proton beams at the National Cancer Center Hospital East

A therapy-dedicated cyclotron was installed in the National Cancer Center H ospital East (NCCHE) at Kashiwa in 1997. Prior to the start of clinical use , we investigated the biological effectiveness of therapeutic proton beams for cell lethality. The proton beams accelerated up to 235 MeV were horizon tally extracted from the cyclotron, and scattered by a bar-ridge filter to produce a Spread-Out-Bragg-Peak (SOBP) of 10-cm width. The biological syste ms used here were mouse intestinal crypt cells and three in vitro cell line s, including SCC61 human squamous cell carcinoma, NB1RGB human fibroblasts and V79 Chinese hamster cells. The dose responses after irradiation at eith er the entrance plateau or the middle portion of SOBP were compared with th ose after linac 6 MV X-ray irradiation. The fit: of a linear quadratic mode l to survival curves showed that proton irradiation increased the cr value of SCC61 and the beta value of V79 cells with a least change for alpha/beta ratio of NB1RGB cells. The isoeffect dose that reduces either cell surviva ls to 10% or mouse jejunum crypts to 10 per circumference was termed D-10. The relative biological effectiveness (RBE) of protons obtained by comparin g the D-10 values between protons and X-rays ranged from 0.9 to 1.2. The de pth distribution of cell lethality was measured by replating V79 cells afte r irradiation from a "cell stack chamber" that received a single dose of 7 Gy at the middle position of SOBP. The thus-obtained cell survivals at vari ous depths coincided well with the estimated survivals, but tended to decre ase at the distal end of SOBP. We conclude that an RBE of 1.1 would be appr opriate for 235 MeV proton beams at the NCCHE.