The relative biological effectiveness (RBE) of carbon ions with linear energy transfer(LET) of 172 keV/μm and 13.7 keV/μm were determined in this study.The clonogenic survivaland premature terminal differentiation were measured on normal human fibroblasts AG01522C andNHDF after exposure of the cells to 250 kV X-rays and carbon ions with different qualities.RBEwas determined for these two biological end points.The results showed that the measured RBE_(10)with a survival fraction of 10% was 3.2 for LET 172 keV/μm,and 1.33 for LET 13.7 keV/μmcarbon ions.RBE for a doubling of post-mitotic fibroblasts (PMF) in the population was 2.8for LET 172 keV/μm,and 1 for LET 13.7 keV/μm carbon ions.For the carbon ion therapy,ahigh RBE value on the Bragg peak results in a high biological dose on the turnout.The tumourcells can be killed effectively.At the same time,the dose on healthy tissue would be reducedaccordingly.This will lighten the late effect such as fibrosis on normal tissue.
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