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By way of introduction
Greetings
By way of self introduction to this list I attach
a brief description of The Human Monitoring Laboratory which
is in Ottawa, Canada.
I look forward to communicating with many of you in the
future on topics of mutual interest.
Gary H. Kramer
The Canadian National Calibration Reference
Centre for In-Vivo Monitoring
Gary H. Kramer
Bureau of Radiation and Medical Devices
Ottawa, Ontario, Canada
K1A 1C1
INTRODUCTION
In Canada the Atomic Energy Control Board (AECB) and the Department of
National Health and Welfare share responsibility for protection of workers
and the public from ionizing radiation. The AECB controls radiation exposures
through its licensing process, under which licensees are required to determine
the radiation dose received by workers and to ensure that the doses are below
the regulatory limits. Doses to the public from licensed facilities are
regulated through environmental, not human, monitoring.
The Department of National Health and Welfare has a general mandate to
protect and preserve the health of Canadians. At its Bureau of Radiation and
Medical Devices (BRMD) a national external dosimetry program, and bioassay
and In-Vivo measurement programs for internal dosimetry are conducted in
addition to the national environmental monitoring programs.
The determination of the radiation dose is generally a two-part process. The
first part is the basic physical measurement such as a whole body count, a
thyroid count or analysis of a thermoluminescent dosimeter which involves the
use of an instrument. The second part of the process converts the
measurement into a dose and is beyond the scope of this presentation.
The first, or measurement, stage requires calibration of the instrument to
ensure an accurate measurement. The calibration process involves
comparison, either directly or indirectly, with an appropriate standard. The
process also requires appropriate documentation to demonstrate that
measurement accuracy is traceable to the standard. Without this traceability,
the estimate of dose received may not be accurate and the hazard to workers
and the public may be increased. Standardization of radiation dose is not only
a regulatory concern but is also a worker and public concern, as evidenced by
the great interest shown by labour unions and by the public.
In 1983, the Atomic Energy Control Board (AECB) decided on the requirement
for a formal program of calibration of all forms of dosimetry. Three working
groups were set up to determine the requirements for comprehensive
calibration programs for external dosimetry, internal dosimetry and dosimetry
of radioactive atmospheres. The Working Group for Internal Dosimetry
recommended that the BRMD administer the calibration reference program for
that aspect of dosimetry as a development of its existing informal calibration
program.
After obtaining funding in 1984 for this development, BRMD established a
reference centre for In-Vivo monitoring. The reference centre provides
calibration sources to Canadian facilities either to confirm that their in-house
calibration is accurate or to provide sufficient information so that the
facility can confidently implement a new calibration data set.
Since 1987 the In-Vivo part of the reference centre has been improving and
expanding the design of the equipment (phantoms) used in the inter-
calibration; recent advances have been greatly influenced by the
recommendations of a Workshop on Standard Phantoms (Kramer and Inn
1991) and the recommendations for Reference Man (ICRP 1975).
Licensees of the Atomic Energy Control Board (AECB), Canada's regulatory
agency, are distributed across the country. To ensure comparability of the
measurement data generated by these organizations, the AECB in 1982
approved a policy requiring all its licensees to calibrate, on a regular basis,
devices and systems used in estimating radiation dose, and to demonstrate the
traceability of such calibrations to AECB-approved national standards. While
suitable standards existed in Canada at the time to meet these requirements
for some licensed operations, e.g., those which made use of gamma-radiation,
there were other operations and areas for which no suitable practical
standards existed. These areas include bioassay, for which the AECB in 1983
designated the Bureau's Bioassay Section and Human Monitoring Laboratory
to act as Canada's National Calibration Reference Centre.
The Centre's mandate was to develop and provide programs and practical
reference standards for measurements used in the assessment of internal dose.
The Working Group, organized to address the technical questions associated
with the development of such standards, recommended that "measurement
traceability" which includes instrument performance, analyst proficiency and
the reliability of procedures used, be the mechanism by which bioassay
laboratories demonstrate their capabilities on a continuing basis to the AECB.
EXISTING PROGRAMS
Presently, the Reference Centre has three In-Vivo intercomparison programs
in place. The In-Vivo programs consist of intercomparisons with whole body
counters, thorax monitoring facilities and thyroid monitoring facilities.
Protocol
The Human Monitoring Laboratory (HML), which conducts the In-Vivo
intercomparison programs uses the BRMD BOMAB phantom family (Kramer
et al. 1991) whole body counting intercomparisons. The BOMAB phantoms,
which are accompanied by HML staff, are used for size dependency
measurements and unknown radionuclide(s) identification. The unknown
radionuclide(s) are not always homogeneously distributed so that the facility
must both identity the radionuclide and its location in the phantom.
Thorax monitoring facilities are usually concerned with lung depositions;
however, they can also be used to monitor other organs of the upper body such
as the liver, abdominal contents or even the head. The HML uses the
Lawrence Livermore National Laboratory Realistic Torso Phantom that has
been recently (Kramer and Inn 1991) accepted as the closest approximation to
a standard torso phantom until such times as a new model is constructed. The
HML holds one LLNL phantom with a large variety of accessory to extend
calibrations to different sized individuals. In addition to the commercially
available radioactive lung sets the HML can also provide other radionuclides
that can be inserted in the blank lung sets or other organs in the torso. For
example, the HML has manufactured some 14C inserts that can be loaded into
the phantom to simulate a lung deposition of insoluble 14C. The phantom is
usually accompanied by HML staff.
A new phantom has been constructed for the Thyroid intercomparison
program. The BRMD neck phantom has been re-designed to remove some of
the shortcomings of other phantoms (Kramer et al. 1990). This phantom is
constructed from a material that is a good tissue substitute for 125I; the form
is anthropomorphic (but no spinal column is included); the thyroid insert
closely resembles the thyroid gland; the depth of the thyroid insert can be
varied; the size of the thyroid insert can easily be modified to simulate small
or large thyroid glands. The BRMD neck phantom is distributed to thyroid
monitoring facilities on request. The phantom is sent with either simulated
125I (129I) or simulated 131I (133Ba + 137Cs), a blank insert, an overlay
plate for multiple depth measurements.
The phantom is also sent with a document
describing its usage (Kramer el al. 1988) as HML staff do not usually
accompany the phantom. Typically the phantom will have two inserts for each
radioiodine. One insert will have an activity in the tens of Becquerels and the
other an order of magnitude higher. The users
are not informed of these activities until after
the inter-comparison is completed.
Performance Criteria
The AECB is in the process of setting
performance criteria for bioassay measurements.
In the interim, the centre uses the limits for the acceptability of results,
which are near the Minimum Detectable Activity (MDA), as shown in the table.
1 to 10 MDA -0.25 < Rel Bias < 0.50
Rel Prec < 0.40
10 to 100 MDA 0.25 < Rel Bias < 0.25
Rel Prec < 0.25
Participants
The participants of the In-Vivo intercomparison program includes nuclear
power generating stations, mining companies, research laboratories,
universities and nuclear medicine departments. Two participants of the
thyroid intercomparison program are from the United States and one
participant is from South America.
DISCUSSION
The Canadian National Calibration Reference Centre for In-Vivo Monitoring
is a unique facility in North America as it provides an external calibration to
a facility to either confirm that their in-house calibration is accurate or
provides sufficient information so that the facility can confidently implement
a new calibration data set. Currently this service is free of charge.
The benefit of the intercomparison program is two-fold. First, facilities can
compare their results to other Canadian facilities and judge their performance
based on the results. Second, and more important, the participation in the
BRMD's intercomparison program allows the facility to demonstrate that their
in-house calibrations are accurate and that their quality assurance program
is performing as expected. The use of an outside independent standard gives
any quality assurance program more credibility than it would otherwise have
if all results were based on in-house data.
FUTURE DIRECTIONS
It is expected that 99mTc will be of regulatory concern in the near future so
that the HML will have to be ready to offer an intercomparison program for this
radionuclide. The HML will continue to refine and improve the existing
phantom families to increase the accuracy of In-Vivo calibrations, this will
result in enhanced safety in the facilities' radiation protection program.
REFERENCES
1. Kramer G. H.; Inn K. G. W. A Summary of the Proceedings of the
Workshop on: Standard Phantoms for In-Vivo Radioactivity
Measurement. Health Physics 1991;61(6): 893-894
2. International Commission on Radiological Protection. Report of the
Task Group on Reference Man. Oxford: Pergammon Press; ICRP
Publication No. 23; 1975.
3. Kramer G. H.; Noel L.; Burns L. The BRMD BOMAB Phantom Family,
Health Physics 1991;61(6): 895-902
6. Kramer G. H.; Gamarnik K.; Noel L.; Burns L. Fabrication of the
Thyroid and Neck Phantoms. Ottawa: Human Monitoring Laboratory;
Human Monitoring Laboratory Technical Document; HMLTD-90-6,
1990.
7. Kramer G. H.; Agterberg D.; Noel L. Thyroid Counter Recalibration
using the BRMD Neck Phantom. Ottawa: Human Monitoring
Laboratory; Human Monitoring Laboratory Technical Document;
HMLTD-88-3, 1988.