Relative Biological Effectiveness Values of Spot-scanning Proton Beam Therapy at Shonan Kamakura General Hospital.

Abstract

This study aimed to confirm the relative biological effectiveness (RBE) values of the proton beam therapy (PBT) system installed in Shonan Kamakura General Hospital. Clonogenic cell-survival assays were performed with a human salivary gland (HSG) cell line, a human tongue squamous-cell carcinoma cell line (SAS), and a human osteosarcoma cell line (MG-63). Cells were irradiated with proton beams and X-rays with different doses (1.8, 3.6, 5.5, and 7.3 Gy for proton beams, and 2, 4, 6, and 8 Gy for X-rays). Proton beam irradiation used spot-scanning methods and three different depths (at the proximal, center, and distal sides of the spread-out Bragg peak). RBE values were obtained from a comparison of the dose that resulted in a surviving fraction of 10% (D10). D10 of proton beams at the proximal, center, and distal sides and X-rays in HSG were 4.71, 4.71, 4.51, and 5.25 Gy, respectively; those in SAS were 5.08, 5.04, 5.01, and 5.59 Gy, respectively; and those in MG-63 were 5.36, 5.42, 5.12, and 6.06 Gy, respectively. The RBE10 values at the proximal, center, and distal sides in HSG were 1.11, 1.11, and 1.16 respectively; those in SAS were 1.10, 1.11, and 1.12, respectively; and those in MG-63 were 1.13, 1.12, and 1.18, respectively. RBE10 values of 1.10-1.18 were confirmed by in vitro experiments using the PBT system. These results are considered acceptable for clinical use in terms of therapeutic efficacy and safety.