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DOE
Lowdose Radiation Program Workshop III
Abstract
_____________________________________________________________________
Title:
Characterizing Bystander Effects Following Electron Microbeam
Irradiation.
Authors:
L.A. Braby and J.R. Ford.
Institutions:
Texas A&M University.
Bystander
effects, which are typically seen as in increase in the cellular
concentration of specific repair related molecules or as cytogenetic
changes which appear to be the consequence of DNA damage,
may be a significant factor in the risk of long-term health
effects of low doses of radiation. These effects clearly increase
the effective size of the target for radiation response, from
the diameter of a single cell or cell nucleus to something
significantly larger, by bringing additional cells into the
process. It is unclear whether this larger target will result
in an increase or a decrease in the probability of inducing
a change which would be detrimental to the health of the organism,
but it clearly reduces the dose below which a linear extrapolation
to zero is required. In order to understand the consequences
of bystander effect on the risk to health from different types
of radiation and different dose rates, it is necessary to
know how the probability of occurrence of these various endpoints
depends on the distance from irradiated cells and on the extracellular
environment in the vicinity of the cells. In order to investigate
these variables for low LET radiation, an electron microbeam
irradiation system has been developed. An electron source
provides a beam with selected energy in the range of about
50 to 100 keV and fluence of about 3 x 1010 electrons cm-2
s-1 to a collimator chosen for the specific experiment.
Cells are grown on the thin plastic film bottoms of specially
prepared Petri dishes and are then irradiated from the bottom
with normal tissue culture medium in place.
Since
the primary objective of this study is to determine the consequences
of cell to cell communication on human tissues, several normal
human bronchial epithelial cell lines have been tested. However,
these cells appear to undergo premature differentiation when
grown in the environment required for microbeam irradiation.
Modifications to the experimental procedures are being tested
in order to develop procedures suitable for quantifying bystander
effects in these cells. Clone 9 cells, which maintain effective
gap junction communication with neighboring cells when grown
in microbeam irradiation conditions, and which show a clear
bystander effect in the induction of PCNA when a fraction
of the cells are exposed to alpha particle irradiation, are
being used for the current electron microbeam irradiation
experiments.
Careful
control of environmental factors is proving to be critical
for the interpretation of low LET microbeam irradiation data.
The condition of the plastic film substrate, as well as the
density of the cells at the time of irradiation, appears to
influence the production of micronuclei. For most of the studies
of low LET irradiation, we have used an irradiation pattern
that delivers a specified number of electrons to each cell
along a line crossing a cluster of cells. In this way, a small
fraction (typically 5%) of the cells receive a specified number
of events, and remainder of the cells is unirradiated.
This research was supported by the Biological and Environmental
Research Program (BER), U. S. Department of Energy, Cooperative
Agreement No. DE-FC03-99ER62858.
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