Purpose: To clarify the relative biological effectiveness (RBE) values of various human cell lines for carbon-ion beams with 2 different linear energy transfer (LET) beams and to investigate the relationship between the cell-killing effect and the biophysical characters, such as the chromosome number and the area of the cell nucleus, using qualitatively different kinds of radiations.
Methods and materials: Sixteen different human cell lines were irradiated with carbon-ion beams, having 2 different LET values (LET(infinity) = 13.3 and approximately 77 keV/microm), accelerated by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences in Japan. Cell-killing effect was detected as reproductive cell death using a colony-formation assay. The number of chromosomes was observed in a metaphase spread using the conventional method. The area of the cell nucleus was calculated as an ellipse on photographs using a micrometer.
Results: The RBE values calculated by the D(10), which is determined as the dose (Gy) required to reduce the surviving fraction to 10%, relative to X-rays, range from 1.06 to 1.33 for 13-keV/microm-beam and from 2.00 to 3. 01 for approximate 77-keV/microm-beam irradiation on each cell line. There was a good correlation in the D(10) values of each cell line between X-rays and carbon-ion beams. However, the D(10) values did not clearly depend on either the chromosome number or the area of the cell nuclei.
Conclusion: The RBE values for HIMAC carbon-ion beams are consistent with previous reports using carbon-ion beams with the similar LET values, and the cellular radiosensitivity of different cell lines well correlate among different types of radiation.