Determining RBE for development of lung fibrosis induced by fractionated irradiation with carbon ions utilizing fibrosis index and high-LET BED model.

•Determining normal tissue tolerance of organs at risk is of utmost importance for the success of carbon ion radiotherapy (CIRT).•This manuscript is the first report on experimental RBE for development of lung fibrosis assessed by quantitative CT after fractionated CIRT.•We further propose RBE estimation by integrating RBEmax and α/βL based modified high-LET BED model.•This model was found to well predict RBE but is sensitive to the uncertainties of α/βL estimates from the reference photon irradiation.•Our findings will contribute to a more precise knowledge of RBE as well as the fractionated dose effects of carbon ion therapy on normal lung tissue.