Office
of Biological and Environmental Research
DOE
Lowdose Radiation Program Workshop III
Abstract
_____________________________________________________________________
Title:
A Variable Energy Soft X-ray Microprobe to Investigate Mechanisms
of the
Radiation Induced Bystander Effect.
Authors:
Melvyn Folkard, Borivoj Vojnovic, Giuseppe Schettino, Kevin
M Prise and Barry D Michael.
Institutions:
Gray Cancer Institute.
We
are currently engaged on two projects in the Low-dose Program:
“Low dose studies with focused X-rays in cell and
tissue models: mechanisms of bystander and genomic instability
responses” (DE-FG07-99ER62877) and “Mechanistic
modeling of bystander effects: An integrated theoretical and
experimental approach” (DE-FG02-02ER63305). Central
to both of these studies is a unique micro irradiation facility
that uses ultrasoft X-rays focused to a sub micron beam for
individual cell and sub cellular targeting. This facility
allows us to selectively irradiate individual cells or sub
cellular locations with high precision and with doses approaching
that of a single electron track. We are concentrating on doses
in the mGy dose range where deviations from a linear dose-effect
relationship are known to occur.
Our
facility uses characteristic X-rays, generated by focused
electron bombardment of a thick target and subsequently focused
to a sub micron spot. To increase the range of energies available
from this source, we are introducing interchangeable targets
made from a range of suitable materials. Our source currently
uses 278eV CK X-rays focused by a zone plate diffraction
lens. Recent technical advances in the design and manufacture
of zone plates now make it possible to efficiently focus X-rays
in the 1-10 keV energy region. The thrust of this proposal
is to extend our X-ray microprobe to take advantage of these
developments such that we have the capability to operate our
microprobe at higher energies. Increasing the range of energies
available is advantageous for a number of reasons. X-rays
with higher energies can have substantially greater penetration,
which leads to a reduced dose variation as the X-ray beam
passes through the cell (CK X-rays are almost fully
attenuated within one cell thickness), and enables cells beyond
the first cell layer to be irradiated when using 3-D tissue
like samples. The ability to irradiate model tissue systems
will allow us to test the hypothesis that the bystander effect
is relevant In vivo. From a microdosimetric point
of view, X-rays with energies greater than a few keV closely
resemble penetrating low LET radiations and by selecting the
X-ray energy, it is possible to vary the radiation quality
of the X-ray microbeam.
We
are in the process of configuring the source for use with
1.48 keV AlK X-rays and in the future, with 4.5
keV TiK X-rays. Underpinning this is the design
and development of zone plates optimized for these energies.
The manufacture of zone plates is highly specialized and we
continue to use established outside sources to supply these
devices. We have already obtained a 0.4 mm diameter zone plate
for use with AlK X-rays that has an effective focal
length of 46 mm (for comparison, our CK zone plate
has a 9 mm focal length). The use of zone plates operating
at higher energies requires that we substantially reconfigure
the geometry of our facility to accommodate changes in focal
length when operating at different energies. Currently, swapping
from using CK X-rays to AlK X-rays is
not straightforward, as it requires dismantling, then rebuilding
the source in a new configuration. A new arrangement is being
designed that will allow rapid substitution of different zone
plate assemblies. Preliminary trails using AlK
X-rays have been highly favorable. Using our existing microfocus
source, we have obtained a dose rate of 2 3 Gy s-1
in the first-order focus (expressed as the mean dose rate
throughout the nucleus of a mammalian cell), with a measured
spot size <500 nm. A new high resolution Si-PIN photodiode
detector (Amptek Inc., Bedford, MA.) has been purchased to
ascertain and optimize the spectral purity of the focused
beam, and to calibrate a proportional counter used for routine
dosimetry. The monoenergetic characteristic X-rays produced
by electron bombardment are accompanied by an unwanted bremsstrahlung
component. By using a grazing incidence mirror between the
source and the zone plate, it is possible substantially to
remove X-ray energies above the desired characteristic energy.
Bremsstrahlung with energies below the characteristic energy
is reduced by filtration. Using the photodiode detector, we
have established that >97% of X-rays in the focus are the
desired AlK X-rays.
Tests
using a titanium target to produce 4.5 keV X-rays have also
been initiated. Optimizing the facility for this energy is
challenging, as a number of factors contrive to reduce the
dose rate to the cells. Nevertheless, there are compelling
reasons that make the availability of focused TiK
X-rays very desirable. One reason is that the penetration
into tissue is considerably greater than can be achieved using
either CK or AlK X-rays (the 1/e attenuation
lengths for CK, AlK and TiK
X-rays are 1.9 mm, 7.0 mm and 170 mm respectively). Our initial
tests indicate that the current source is capable of delivering
up to 3.6x104 TiK X-rays per second through a 0.5
mm square silicon nitride window, just prior to the zone plate
assembly. Calculations show that this corresponds to ~2000
TiK X-rays per second in the first order focus
of an ideal zone plate. This is a low dose rate (several minutes
to deliver 1 Gy) compared to CK and AlK
X-rays, but should be sufficient for bystander type studies.
Nevertheless, it is our intention to develop a new high output
X-ray source to substantially increase the output at all energies.
Return
to the top
