Purpose: To determine the number of initial chromatid breaks induced by low
- or high-LET irradiations, and to compare the kinetics of chromatid break
rejoining for radiations of different quality.
Material and methods: Exponentially growing human fibroblast cells AG1522 w
ere radiated with gamma-rays, energetic carbon (290 MeV/u), silicon (490 Me
V/u) and iron (200 and 600 MeV/u). Chromosomes were prematurely condensed u
sing calyculin A. Chromatid breaks and exchanges in G2 cells were scored. P
CC were collected after several post-irradiation incubation times, ranging
from 5 to 600 min.
Results: The kinetics of chromatid break rejoining following low- or high-L
ET irradiation consisted of two exponential components representing a rapid
and a slow time constant. Chromatid breaks decreased rapidly during the fi
rst 10 min after exposure, then continued to decrease at a slower rate. The
rejoining kinetics were similar for exposure to each type of radiation. Ch
romatid exchanges were also formed quickly. Compared to low-LET radiation,
isochromatid breaks were produced more frequently and the proportion of unr
ejoined breaks was higher for high-LET radiation.
Conclusions: Compared with gamma-rays, isochromatid breaks were observed mo
re frequently in high-LET irradiated samples, suggesting that an increase i
n isochromatid breaks is a signature of high-LET radiation exposure.