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RADIATION PROTECTION IN NUCLEAR MEDICINE
PART 11. POTENTIAL EXPOSURE
1.
POTENTIAL EXPOSURE, SAFETY ASSESSMENT
Not all exposures occur as expected. Accidental exposure may occur due to
equipment failure, human error or a combination of both. Although such events can
be anticipated by a careful safety analysis, their details and the time of occurrence
can not be predicted. These exposures are called potential exposures. The Basic
Safety Standards define potential exposure as: “Exposures that may or may not be
delivered and to which a probability of occurrence can be assigned”
To deal with potential exposures, it is necessary to perform a safety
assessment to:
 identify what can go wrong ,
 evaluate if the preventive measures are adequate, and
 prepare contingency and emergency plans to mitigate the consequences of
accidents, should they occur .
The licensee shall conduct a safety assessment applied to all stages of the
design and operation of the nuclear medicine facility, and present the report to the
Regulatory Authority if required.
The safety assessment shall include, as
appropriate, a systematic critical review of the identification of all possible events
leading to an accidental exposure (BSS IV.3–7).
The safety assessment should not only cover these events, but make an
effort to anticipate other events that have not previously been reported.
The safety assessment shall be documented and, if appropriate,
independently reviewed by an expert, within the QA programme. Additional reviews
shall be performed as necessary whenever:
 safety may be compromised as a result of modifications of the facilities or of
the procedures;
 operational experience or information on accidents or errors indicates that a
review is necessary; or
 any significant changes to relevant guidelines or standards are envisaged or
have been made.
Any consequential modifications shall be made cautiously and only after a favourable
assessment of all the implications for protection and safety.
Consideration should be given to potential exposures in the prior radiological
evaluation of all aspects of the operation to identify both the routine and the
reasonably foreseeable potential sources of exposure. Basically there should be two
objectives, prevention and mitigation. Prevention is the reduction of the probability
of the sequence of events that may cause or increase radiation exposures. It involves
maintaining the reliability of all the operating and safety systems and of the
associated working procedures. Mitigation is the limitation and reduction of the
exposures if any of these sequences do occur. It involves the use of engineered
safety features and operational procedures to control each sequence of events with
the aim of limiting any consequences, should an exposure occur.
On the basis of events identified by the safety assessment, the licensee shall
prepare emergency procedures (BSS V.2–6). The procedures should be clear,
concise and unambiguous and shall be posted visibly in places where their need is
anticipated. An emergency plan shall, as a minimum, list/describe:
 predictable incidents and accidents and measures to deal with them;
 the persons responsible for taking actions, with full contact details;
 the responsibilities of individual personnel in emergency procedures (nuclear
medicine physicians, medical physicists, nuclear medicine technologists,
etc.);
 equipment and tools necessary to carry out the emergency procedures;
 training and periodic rehearsal;
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PART 11. POTENTIAL EXPOSURE
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recording and reporting system;
immediate measures to avoid unnecessary radiation doses to patients, staff
and public;
measures to prevent access of persons to the affected area; and
measures to prevent spread of contamination.
In nuclear medicine the primary hazard from most sealed and unsealed
radionuclide sources is that of external radiation by gamma rays. A secondary hazard
could arise from internal radiation following accidental ingestion of the activity. If a
serious spillage of radioactive material were to occur then both the shielding and the
containment of the source could be lost.
If a spillage of radioactive material were to occur then both the shielding and
the containment of the source could be lost. The working practices described in the
Local Rules should be followed at all times in order to prevent a spillage occurring
and to minimize the consequences if one should occur.
An accident involving damage to a 99Mo/99mTc radionuclide generator that
resulted in the release of 99Mo would be one of the most serious foreseeable
accidents that could occur in most nuclear medicine departments. If leakage of 99Mo
from a generator is suspected the immediate area should be evacuated immediately
and the radiation protection officer must be informed.
Accidents and incidents involving patients are described below.
In the event of a fire the normal hospital procedures should be observed. The
safe evacuation of all patients, visitors and staff is the most important consideration.
When the fire department arrives they should be informed of the presence of
radioactive sources in the building. No one should be permitted to re-enter the
building until it has been checked for contamination and re-entry approved by the
radiation protection officer.
In the event of an incident or accident, a comprehensive description of the
incident should be prepared including the following information:
 a description of the incident by all persons involved,
 methods used to estimate the radiation dose received by those involved in the
incident and implications of those methods for possible subsequent litigation,
 methods used to analyze the incident and to derive risk estimates from the data,
 the subsequent medical consequences for those exposed,
 the particulars of any subsequent legal proceedings that may ensue,
 conclusions drawn from the evaluation of the incident and recommendations on
how to prevent a recurrence of such an accident.
2.
ACCIDENT PREVENTION, LESSONS LEARNED
The licensee shall incorporate within the RPP (BSS IV.10–12):
 defence in depth measures to cope with identified events, and an evaluation
of the reliability of the safety systems (including administrative and
operational procedures, and equipment and facility design);
 operational experience and lessons learned from accidents and errors. This
information should be incorporated into the training, maintenance and QA
programmes;
The licensee shall promptly inform the Regulatory Authority of all reportable events,
and make suitable arrangements to limit the consequences of any accident or
incident that does occur.
It must be emphasized that the best way to avoid radiation accidents is to
have a structured radiation protection framework, together with associated quality
assurance and quality control programmes. A safety culture should also have been
implemented in the organization. A safety culture should include collection of
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PART 11. POTENTIAL EXPOSURE
information on unusual events, which led or might have led to incidents and
accidents. This information provides material that can be used to prevent future
accidents.
Examples of accidents and incidents that have led to unintended medical
exposure include treatment or examination of the wrong patient with the wrong
activity and the wrong radiopharmaceutical as well as treatment or examination of
pregnant or lactating female patients. Identified causes of these kinds of accidents
and incidents include:
 Communication problems
 Busy environment, distraction
 Unknown local rules
 No training in emergency situations
 Not clearly defined responsibilities
 No efficient quality assurance
In order to avoid future accidents it is important to learn from previous ones. The
initiating event and the contributing factors can always be identified. This information
provides material that should be used to prevent future accidents. This is achieved by
having an efficient reporting system and a programme for continuous education and
training.
3.
REFERENCES
1.
INTERNATIONAL ATOMIC ENERGY AGENCY. International Basic Safety
Standards for Protection Against Ionizing Radiation and for the Safety of
Radiation Sources. Safety Series No.115, IAEA, Vienna (1996).
2.
INTERNATIONAL ATOMIC ENERGY AGENCY. Model Regulations on
Radiation Safety in Nuclear Medicine. (in preparation).
3.
INTERNATIONAL ATOMIC ENERGY AGENCY. Regulations for the Safe
Transport of Radioactive Material. Safety Series No. ST-1, IAEA, Vienna
(1996).
4.
INTERNATIONAL ATOMIC ENERGY AGENCY. Safety Assessment Plans
for Authorizations and Inspection of Radiation Sources. IAEA-TECDOC-1113,
IAEA, Vienna (1999).
5.
INTERNATIONAL ATOMIC ENERGY AGENCY, Generic Procedures for
Assessment and Response During a Radiological Emergency.
IAEATECDOC-1162, IAEA, Vienna (2000).
6.
INTERNATIONAL ATOMIC ENERGY AGENCY. Safety Report on
Methodology for Investigation of Radiation Accidents IAEA, Vienna (in press).
7.
INTERNATIONAL ATOMIC ENERGY AGENCY. Lessons learned from
accidents and errors in radiotherapy. IAEA, Vienna, 1997.
8.
PAN AMERICAN HEALTH ORGANIZATION. Organization, development,
quality control, and radiation protection in radiology services. PAHO
Washington D.C., (1997).
9.
NATIONAL
COUNCIL
ON
RADIATION
PROTECTION
AND
MEASUREMENTS Developing radiation emergency plans for academic,
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PART 11. POTENTIAL EXPOSURE
medical or industrial facilities. NCRP, Bethesda, 1991 (NCRP Report No.
111).
10.
SERIS D. Human Factors and the Medical Use of Nuclear Byproduct
Material. Proceedings of the Human Factors Society 33rd Annual Meeting,
1989
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