Dose Distribution Degradation of Carbon-ion Radiotherapy Caused by Tumor Cell-specific Relative Biological Effectiveness of Osteosarcoma: A Simulation Study Using Experimental Results.

Background/aim: Dose distributions of carbon-ion radiotherapy (C-ion RT) have been created with the relative biological effectiveness (RBE) of human salivary gland cells (HSG). However, no dose distributions have been created using various tumor cell-specific RBE values. Hence, we conducted in vitro experiments to determine the RBE of human osteosarcoma cells (U2OS) and used this RBE value (RBE) to calculate the dose distribution for C-ion RT.

Materials And Methods: To obtain RBE values for various linear energy transfer (LET) levels, we exposed U2OS cells to different doses of X-rays and varying doses and LET levels of C-ion beams (13, 30, 50, and 70 keV/μm). Subsequently, we converted the RBE of HSG (RBE) to RBE in the treatment planning system and reconstructed the dose distribution for a typical osteosarcoma case. We performed a dose-volume histogram (DVH) analysis, evaluating the percentage of the minimum dose that covered 98%, 50%, and 2% (D, D, and D, respectively), as well as the homogeneity index [HI; calculated as (D-D)/D].

Results: The RBE values for C-ion beams with LET of 13, 30, 50, and 70 keV/μm were 1.77, 2.25, 2.72, and 4.50, respectively. When comparing DVH parameters with the planning target volume, we observed the following values: D, D, D, and HI for RBE were 64.1, 70.1, 72.4 Gy (RBE), and 0.12, respectively. For RBE, these values were 86.2, 95.0, 107.9 Gy (RBE), and 0.23, respectively.

Conclusion: We utilized RBE to calculate the dose distribution of carbon ion radiotherapy, revealing potential degradation in dose distribution and particularly worsening of the HI.