to develop dosimetric
standards for x rays, gamma rays, and electrons
based on the SI unit, the gray,
for homeland security, medical,
radiation processing, and radiation protection applications.
INTENDED OUTCOME AND BACKGROUND
The Radiation Interactions and
Dosimetry Group promotes the accurate
and meaningful measurements of dosimetric
quantities pertaining to ionizing
radiation: x rays, gamma rays, electrons,
and energetic, positively charged particles.
We maintain the national measurement
standards for the Système
International (SI) unit for radiation dosimetry, the gray.
NIST is a world leader in the measurement
of high levels of absorbed dose, as
required in the industrial radiation processing
of materials (e.g., sterilization
of single-use medical devices, food irradiation,
and destruction of biological
weapons). Accurate transfer dosimetry
is increasingly done on the basis of
alanine/EPR (Electron Paramagnetic
Resonance) dosimetry, rather than
radiochromic film dosimetry, which was
originally developed at NIST and
offered for many years as a calibration
service. A new NIST system is near
completion for on-demand, Internet-based
e-calibrations for industry, based on alanine/EPR dosimetry.
Brachytherapy (treatment with sealed
radioactive sources) has seen a tremendous
increase in the use of low-energy,
photon-emitting seeds to treat prostate
cancer and in the use of beta-particle-
(and photon-) emitting sources to inhibit
arterial restenosis (re-closing) following
balloon angioplasty. In both cases,
NIST has responded to the needs of the
manufacturers, regulators, and clinical
physicists. We develop new standards
and measurement methods to calibrate
the quantities needed to ensure accurate
dosimetry for the wide variety of sources
introduced, and we disseminate these
standards through a network of secondary
calibration laboratories.
Nearly 700,000 cancer patients per year
are treated in the U.S. with radiation
beams, mainly from high-energy electron
accelerators (either directly with
the electrons or by converting them to
high-energy x rays). NIST maintains
and disseminates the standards for air
kerma (exposure) and for absorbed dose
to water from 60Co gamma-ray beams.
These provide the basis for calibrating
instruments used to measure the
absorbed dose delivered in therapy beams.
Standards for diagnostic radiology are
developed and maintained at NIST in
terms of air kerma, for x-ray beams from
10 kV to 300 kV (x-ray source accelerating
potential in kilovolts). These are disseminated
to manufacturers and the
medical physics community in North
America through a network of secondary
calibration laboratories. NIST maintains
some 85 beam qualities for conventional,
W-anode, x-ray beams, and
17 beam qualities for mammographic,
Mo- and Rh-anode, x-ray beams.
The radiation transport and Monte
Carlo methods pioneered and developed
at NIST to calculate the penetration of
electrons and photons in matter are
used in most of the major codes today.
Monte Carlo simulation is increasingly
applied to problems in radiation metrology,
protection, therapy, and processing
as an accurate tool for designing and
optimizing radiation systems and for
providing important insight into
processes inaccessible to measurement.
Accomplishments
Assistance to U.S. Postal
Service in Decontamination
of Mail
After anthrax-laced mail was delivered
to media and government offices, resulting
in five deaths, numerous illnesses,
and enormous disruption and economic
loss, we responded rapidly to identify
industrial irradiation of the mail as an
effective and readily available process to
kill anthrax spores. Leading a task force
established by the White House Office
of Science and Technology Policy, we
worked with the Armed Forces
Radiobiology Research Institute, the
U.S. Postal Service (USPS), and industrial
irradiation facilities to provide
critical dosimetry measurements and
to validate the process.
Based on an extensive program of
Monte Carlo radiation transport calculations
and accurate dosimetry measurements
in a variety of mail configurations,
NIST provided advice for optimizing
the process parameters and
developing a national strategy to
effectively handle the highly variable
mail and parcel stream.
This collaboration continues, expanded
to include qualitative measurements of
radiolytic products produced in the mail
during the irradiation, quantitative
measurements of the effects on the
archival properties of paper due to irradiation,
and the design of a dedicated
USPS mail-irradiation facility.
High-Energy Computed
Tomography Facility
The 7 MeV to 32 MeV Saggataire
electron linac in the NIST Medical
Industrial Radiation Facility (MIRF)
offers unique possibilities as an x-ray
source for a high-energy computed
tomography (HECT) facility. A beamline
and camera are under development
at MIRF to study the x-ray inspection
of cargo containers, trucks, and other
large objects.
We are also carrying out theoretical
and experimental investigations into
neutron production in high-energy x-ray
beams. It might prove possible to also
use photoneutrons, produced at high
photon energies, as an active probe to
interrogate containers and screen for
explosives and other terrorist materials.
Radiation Sources for Medical and Industrial Applications
| |
Figure 5. NIST medical-therapy accelerator
facility.
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In addition to a dozen gamma-ray
(60Co and 137Cs) sources and five x-ray
ranges, we maintain MIRF, a 4 MeV
electron Van de Graff, a 500 keV electrostatic
electron accelerator, and a 1 MeV to 8 MeV
pulsed electron linac. These are used in a variety of
radiation applications such as material modification,
radiation-hardness testing, electron-
and bremsstrahlung-beam dosimetry, and HECT development.
New accelerator facilities, incorporating
machines currently used in medical
and industrial applications, are bringing
our programs to the forefront of
research and development. Installation
of a 6 MeV to 20 MeV electron-beam
(6 MV and 18 MV bremsstrahlung-beam)
Varian Clinac 2100C to support
the development of direct, therapy-level
dosimetry calibrations has been
completed. (See Fig. 5.)
Planning is underway for a facility based
on two Titan 10 MeV, 17 kW electron
linacs, donated by the U.S. Postal
Service, to support the standards and
calibrations program for homeland
security applications, general industrial
radiation processing, and the study of
radiation effects in materials.
Calibration of Low-Energy Photon Brachytherapy Sources
| |
Figure 6. Close-up view of a seed (illuminated
by a laser), mounted vertically on a rotating
post, ready for calibration on the WAFAC calibration range.
|
Small radioactive "seed" sources used in
prostate brachytherapy, containing the radionuclide
103Pd, 125I, or 131Cs,
are calibrated in terms of air-kerma strength
using the NIST Wide-Angle Free-Air
Chamber (WAFAC). The WAFAC is an
automated, free-air ionization chamber
with a variable volume, allowing
corrections to be made for passage of
the beam through non-air-equivalent
electrodes. Over 500 seeds of 32 different
designs, from 18 manufacturers,
have been calibrated using the WAFAC
since 1999. (See Fig. 6.)
On-site characterization at seed manufacturing
plants for quality control, as
well as at therapy clinics for treatment
planning, relies on well-ionization
chamber measurements. Following the
primary standard measurement of air-kerma
strength, the responses of several
well-ionization chambers to the various
seed sources are determined. The ratio
of air-kerma strength to well-chamber
response yields a calibration coefficient
for the well-ionization chamber for a
given seed type. Such calibration coefficients
enable well-ionization chambers
to be employed at therapy clinics for
verification of seed air-kerma strength,
which is used to calculate dose rates to
ensure effective treatment planning.
To verify that seeds of a given design
calibrated at NIST are representative of
the majority of those calibrated in the
past, several additional tests have been
implemented. The distribution of
radioactive material within a seed is
mapped using radiochromic film contact
exposures. The in-air anisotropy of
seeds is studied by taking WAFAC and
x-ray spectrometry measurements at discrete
rotation angles about the long axis
and the axis perpendicular to the midpoint
of the long axis of the seed,
respectively. The "air-anisotropy ratio,"
calculated from the results of angular
x-ray measurements, has proven to be
a useful parameter for explaining differences
in well-chamber response observed
for different seed models having the
same emergent spectrum on their transverse axis.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
