THE RBE OF FAST-NEUTRONS FOR IN-VITRO INACTIVATION OF HUMAN TUMOR-CELLS DETERMINED BY THE RATIO OF MEAN INACTIVATION DOSES

In an effort to clarify the relationship between sensitivity of human tumour cells to low-LET and to fast neutron irradiation, 10 human tumo ur cell lines were exposed to cobalt gamma-rays and to 60 MeV (p-->Be) neutron beam, The data were pooled with results of 31 human tumour c ell lines previously published, The analysis of data using the linear- quadratic model indicated that not only alpha values increased after n eutron irradiation, but so did beta values too, although to a lesser e xtent, The mean inactivation dose (MID) was derived for each cell line from the linear-quadratic parameters after low-LET and high-LET expos ure, MID values following neutron irradiation were closely correlated to those after gamma-ray irradiation. In these 41 cell lines, the extr eme 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, mea ning that intrinsically radiation resistant tumour cells have a higher neutron RBE, on average, Similar findings were observed if alpha rati os were used instead of MID ratios, In addition, the RBE/dose variatio ns were more marked in cells with the higher RBE. Taken together, thes e data suggest that, although considerable variations exist among huma n tumour cell lines, intrinsically radioresistant cells are relatively more sensitized when exposed to high LET beams than radioresponsive t umours, An 'intrinsic gain factor' may thus be expected in irradiating radiation resistant tumours with fast neutrons, in addition to the hy poxic or kinetic gain factors, Because the quadratic component is stil l present after neutron irradiation, we suggest using MID ratio as a r eference RBE when comparing survival curves of cells exposed to radiat ions of different qualities.