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 cells to low-LET and to fast neutron irradiation, 10 human tumour 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 cell lines previously published. The analysis of date using the linear-quadratic model indicated that not only alpha values increased after neutron irradiation, but so did beta 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 observed if alpha ratios were used instead of MID ratios. In addition, the RBE/dose variations were more marked in cells with the higher RBE. Taken together, these data suggest that, although considerable variations exist among human tumour cell lines, intrinsically radioresistant cells are relatively more sensitized when exposed to high LET beams than radioresponsive tumours. An 'intrinsic gain factor' may thus be expected in irradiating radiation resistant tumours with fast neutrons, in addition to the hypoxic or kinetic gain factors. Because the quadratic component is still present after neutron irradiation, we suggest using MID ratio as a reference RBE when comparing survival curves of cells exposed to radiations of different qualities.
Courdi
A
A
Centre Antoine-Lacassagne, Biomedical Cyclotron, Nice, France.
Brassart
N
N
Herault
J
J
Mari
D
D
Chauvel
P
P
D002470:Cell Survival / Q000528:radiation effects; D005214:Fast Neutrons / Q000627:therapeutic use*; D006801:Humans; D009369:Neoplasms / Q000235:genetics / Q000532:radiotherapy*; D011003:Ploidies; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness; D014407:Tumor Cells, Cultured