The RBE of Fast Neutrons for in Vitro Inactivation of Human Tumour Cells Determined by the Ratio of Mean Inactivation Doses

In an effort to clarify the relationship between sensitivity of human tumour ceUs to low-LET and to fast neutron irradiation, 10 human tumour cell lines were exposed to cobalt gamma-rays and to 60MeV (p→Be+) neutron beam. The data were pooled with results of 31 human tumour cell lines previously published. The analysis of data using the linear-quadratic model indicated that not only a values increased after neutron irradiation, but so did B values too, although to a lesser extent. The mean inactivation dose (MID) was derived for each cell line from the linear-quadratic parameters after low-LET and high-LET exposure. MID values following neutron irradiation were closely correlated to those after gamma-ray irradiation. In these 41 cell lines, the extreme values of RBE derived by the ratio of MID varied by a factor of 3 among the cell lines. RBE was positively correlated to photon MID, meaning that intrinsically radiation resistant tumour cells have a higher neutron RBE, on average. Similar findings were obs...