Investigation of the relative biological effectiveness and uniform isobiological killing effects of irradiation with a clinical carbon SOBP beam on DNA repair deficient CHO cells.

Spread-out Bragg peak (SOBP) C ions have been used clinically in charged particle radiation therapy for years. An SOBP beam consists of various monoenergetic Bragg peaks; however, the biological effect of irradiation with an SOBP beam track has not been well-studied. In order to determine the clinically prospective molecular targets, radiosensitivity to the beam track in DNA repair deficient cell lines was investigated. A total of four distinct Chinese hamster ovary (CHO) cell lines, including CHO10B2 (wild-type), V3 (protein kinase DNA-activated catalytic polypeptide deficient), 51D1 (RAD51 paralog D deficient) and PADR9 [poly(ADP-ribose) polymerase (PARP) deficient], were irradiated with gamma-rays, C ions (290 MeV/n) and Fe ions (500 MeV/n), in order to compare cellular lethality. An OptiCell™ culture system was used to evaluate the lethality at distinct depths of SOBP C ions. Relative biological effectiveness (RBE) values of C ions (linear energy transfer (LET), 13 and 70 keV/µm) and Fe ions (LET, 200 keV/µm) were calculated from cell survival using colony formation assay with standard cell dishes. All cell lines exhibited similar sensitivity to 70 keV/µm C ions and 200 keV/µm Fe ions. Furthermore, V3 cells did not exhibit increased sensitivity to high LET C ions and Fe ions, compared with low LET gamma-rays and C ions, and 51D1 cells irradiated with 13 keV/µm C ions exhibited relatively high RBE values among the tested cell lines. Conversely, PADR9 cells exhibited low RBE values for 13 keV/µm C ions and high RBE values for 70 keV/µm C ions. Obtained using the OptiCell system, the survival fractions in the SOBP region were uniform for wild-type and PADR9 cells. Conversely, V3 and 51D1 cells exhibited decreased cell death in the distal region of the SOBP. These results indicated that PARP is a more effective target for clinical beam therapy, compared with the non-homologous end joining repair and homologous recombination repair pathways. PARP deficiency may be an optimal target for C ion therapy and the results of the present study may contribute to the development of a more effective heavy ion radiation therapy.