Human melanoma cells that are resistant to gamma rays were irradiated with
14 MeV neutrons given at low doses ranging from 5 cGy to 1.12 Gy at a very
low dose rate of 0.8 mGy min(-1) or a moderate dose rate of 40 mGy min(-1).
The biological effects of neutrons were studied by two different methods:
a cell survival assay after a 14-day incubation and an analysis of chromoso
mal aberrations in metaphases collected 20 h after irradiation. Unusual fea
tures of the survival curve at very low dose rate were a marked increase in
cell killing at 5 cGy followed by a plateau for survival from 10 to 32.5 c
Gy. The levels of induced chromosomal aberrations showed a similar increase
for both dose rates at 7.5 cGy and the existence of a plateau at the very
low dose rate from 15 to 30 cGy. The existence of a plateau suggests that a
repair process after low-dose neutrons might be induced after a threshold
dose of 5-7.5 cGy which compensates for induced damage from doses as high a
s 32.5 cGy. These findings may be of interest for understanding the relativ
e biological effectiveness of neutrons and the effects of environmental low
-dose irradiation. (C) 2000 by Radiation Research Society.