60. Difference in LET-RBE Spectra for Cell Killing Exposed to Accelerated Ion Beams

Yoshiya Furusawa, Mizuho Saito¹ Kumiko Fukutsu, Hiromi Itsukaichi, Kiyomi Eguchi-Kasai, Tatsuaki Kanai, Koichi Ando, Fumio Yatagai² Marcelo E. Vazquez³, Tracy. Yang⁴ and Hiroshi Ohara⁵ (¹Chiba Univ.; ²Inst. Phys. Chem. Res. ³Brookhaven Natl. Lab. ⁴NASA Johnson Space Center; ⁵Okayama Univ.)

Keywords: heavy ion beam, LET, RBE

The relationship between the radiobiological effectiveness (RBE) of cell killing and linear energy transfer (LET) was obtained using V79 cells for ²⁸Si-, ⁴⁰Ar-, and ⁵⁶Fe-ion beams having LET among 55-800, 85-1500, and 150-6000 keV/m, respec tively, using HIMAC (NIRS, Chiba), RRC (RIKEN Wako), and AGS (BNL, New York).

Cell survival curves and their parameters were obtained by a colony-forming assay and curve-fitting using the LQ-equation. The LET-RBE relationship for each ion could be well-fitted by a newly established fitting function to LET with 3 constants, i. e. maximum RBE, LET giving maximum RBE, and width of the peak. The maxim RBEs were found at around 200 keV/m, and the values were 4.7 3.7 for Si-, Ar-, and Fe-ions. By increasing the particle atomic number, we found: a) a shift of the peak RBE towards a higher LET region, b) a decrease in the maximum RBE, and c) a less sharp fall-off of the RBE at very high LET. These data agree with previous results obtained with ³He-,¹²Cand ²⁰Neions, and point out a general behavior for splitting of RBE-LET curve for particles with different atomic number Z or mass number A.

The LET-RBE spectra on cell killing for cultured mammalian cells exposed to accelerated heavy ions were investigated in order to design a spread-out Bragg peak beam for cancer therapy at HIMAC prior to clinical trials. Human salivary-gland tumor originated HSG cells as well as V79 and Tl cells were exposed to ³He¹²Cand ²⁰Ne-ion beams with the LET ranging over approximately 20 600 keV/ m under both aerobic and hypoxia conditions. Cell survival curves were fitted by equations from the LQ-model and the target-model to obtain survival parameters. RBE, OER, and D0, were analyzed as a function of the LET. The RBE increased with LET showing the maximum at around 200 keV/ m, then decreased with LET. Clear splits of the LET-RBE or -OER spectrum were found among ion-spices and/ or cell lines. The RBE for a ³He-beam was higher than Chat of heavier ions, the LET that showed maximum RBE shifted to a higher LET region for heavier ions. The OER value was 3 for X-rays, but started to decrease at a LET of around 50 keV/ m, crossed 2 at around 100 keV/ m, and then reached the minimum above 300 keV/ m, though the values were greater than 1. The OER was significantly lower for ³He-ions than others.

Publications:
1)Durante, M., Furusawa, Y., Vlajima, H., Gotoh, E.: 1111 Risk Evaluation of Cosmic-Ray Exposure in Long-Term Manned Space Mission. Fujitaka, K., et al. (eds.) Kodansha Scientific Lid., Tokyo, pp 71-81,1999.
2)Ohara, H., Okazaki, N., Monobe, M., Watanabe, S. Minamihisamatsu, M., Furusawa, Y., Kanau, T.: In; Risk Evaluation of Cosmic-Ray Exposure in Long-Term Manned Space Mission. Fujitaka, et al. (eds.), Kodansha Scientific Lid., Tokyo, pp. 101 108,1999.
3)Vazqurz, M.F., Saito, M., Nojima, K., Furusawa, Y., Pelzer, A.: Proceedings of 10th Annual Space Radiation Health Investigators Workshop. Brookhaven National Laboratory, New York, 1999.5.
4)Furusawa, Y., Saito, M., Kanai, T., Yatagai, F Vazquez, M., and Yang, TC. : International Congress of Radiation Research, Dublin, 1999.