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RADIATION PROTECTION IN NUCLEAR MEDICINE
PART 12. PUBLIC EXPOSURE
1.
DOSE LIMITS
According to the Basic Safety Standards, for radiation protection purposes, a
member of the general public is considered to be anyone who is not occupationally
exposed or a patient undergoing a dental radiographic examination or someone
knowingly and voluntarily helping (other than in their employment) in the care,
support or comfort of a patient. The exposure of the general public is restricted by
the application of dose limits and the constrained optimization of radiation protection.
These dose limits do not apply to radiation exposures from natural sources.
The Standards state that: “ The estimated average doses to the relevant
critical groups of members of the public that are attributable to practices shall not
exceed the following limits:
(a) an effective dose of 1 mSv in a year;
(b) in special circumstances, an effective dose of up to 5 mSv in a single year,
provided that the average dose over five consecutive years does not exceed 1 mSv
per year;
( c) an equivalent dose to the lens of the eye of 15 mSv in a year; and
(d) an equivalent dose to the skin of 50 mSv in a year.”
The Regulatory Authority or the radiation protection officer may establish a
dose constraint which is usually a fraction of the annual public dose limit governing
the radiation dose from a specific practice that may be received by visitors or hospital
workers not directly working with radioactive materials. An example of such a dose
constraint is 0.3 mSv per procedure.
2.
DESIGN CONSIDERATIONS
The likelihood of unnecessary exposure of the public (i.e. visitors or anyone not
authorized to receive occupational radiation doses) may be reduced by the following
measures.
The movement of radionuclides must be minimized. For example, in a
nuclear medicine department the dose preparation and administration room should
be adjacent and connected by a pass through. In a research facility, the radiation
measuring equipment should be close to the laboratory where the radionuclides are
used.
Areas where significant activities of radionuclides are present must be
appropriately shielded. Radionuclides must be stored in shielded containers and
shielded barriers erected if necessary. Judicious positioning of such areas in relation
to public access areas must also be considered.
Access must be restricted so that members of the public are not allowed into
controlled areas. In a nuclear medicine facility, a separate waiting area and toilet
facilities are advised for injected patients. Only laboratory staff should be permitted
in the room when radionuclides are being handled and the doors and windows
should be lockable. Radioactive waste must be stored in a secure location away
from areas accessible to the public.
Every precaution must be taken to ensure that the doses received by
individuals who come close to a patient or who spend some time in neighbouring
rooms remain below the dose limit for the public and below any applicable dose
constraint
3.
THE RADIOACTIVE PATIENT
The activities administered for diagnostic purposes are moderate, and the patient
does not normally need to be hospitalized. For almost all diagnostic procedures the
maximum dose that could be received by another person due to external exposure
from the patient is a fraction of the annual public dose limit and it should not normally
be necessary to issue any special radiation protection advice to the patient's family.
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PART 12. PUBLIC EXPOSURE
The management of patients undergoing radionuclide therapy is designed to
minimize radiation exposure to other persons.
The precautions to be taken for ambulatory patients treated for thyrotoxicosis
depends upon the amount of radioactivity administered, the radiation dose rate in the
vicinity of the patient, expected normal patterns of daily contact between the patient
and others, and dose constraints that may apply.
All patients should be advised of basic hygiene measures (toileting, handwashing, etc) to minimize contamination of their home and work environment.
Females should avoid pregnancy for at least 6 months following therapy, in case
follow-up examinations might discover the need for further radiation therapy; and
males should take precautions for two months to avoid beginning a pregnancy. In
addition, nursing mothers undergoing radioiodine therapy are advised that complete
cessation of breast-feeding is necessary
As stated in the Basic Safety Standards :”In order to restrict the exposure of
any members of the household of a patient who has undergone a therapeutic
procedure with sealed or unsealed radionuclides and members of the public, such a
patient shall not be discharged from hospital before the activity of radioactive
substances in the body falls below the level specified...”.
At the present time only iodine -131 has been considered as needing this
precaution and a guidance level of 1000MBq has been given in the Standards.
However, in some countries a level as low as 400MBq is considered good practice.
In addition as mentioned in the Standards, “Written instructions to the patient
concerning contact with other persons and relevant precautions for radiation
protection shall be provided as necessary”.
After patient discharge, the radiation protection officer should supervise the
removal of any contaminated waste, the decontamination of the room and
equipment; and should conduct a documented final survey of the room. When the
survey and decontamination procedures are complete, the RPO should remove the
radiation warning sign from the room door and notify nursing and housekeeping
departments that the room is now clear for general use.
4.
SPECIAL PROBLEMS
Occasionally the condition of a patient treated with radionuclides may lead to death
while the body still contains substantial residual activity. The rules and regulations
governing the embalming, burial, or autopsy of cadavers containing substantial
residual activities vary from country to country since they take account of various
social, climatic, and religious factors.
When a living patient is released from the hospital with significant
incorporated amounts of radioactive material, the physician in charge must make
certain that appropriate instructions are given to the relatives in the event of death.
The most important instruction should be to contact the nuclear medicine department
as soon as possible, and to delay any decisions about proceeding with arrangements
for autopsy or disposal of the corpse until expert advice has been obtained.
In the event of the death of a patient who has recently received a therapeutic
dose of a radionuclide care has to be taken to ensure that workers and the public
receive as low a dose as reasonably achievable at all stages prior to the burial or
cremation. A contingency plan should be described in written instructions to the
hospital staff to be implemented for radionuclide therapy patients who die while still in
the hospital.
The nuclear medicine department should be consulted immediately to
determine (by direct measurement or calculation) the amount of residual radioactivity
in a cadaver. If there is still significant residual radioactivity, the physician who
declares the patient dead should attach a label to the body indicating the presence
and amount of radioactivity and the radionuclide. The label should be readily
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PART 12. PUBLIC EXPOSURE
recognizable, legible, and easy to understand. The precautions to be taken in
handling such cadavers depend on the nature and quantity of the radionuclide
present and on the type of handling intended (e.g., autopsy or merely simple
treatments prior to burial). As a general rule it can be said that no appreciable hazard
exists unless the body is opened, when the hands and face of the pathologist could
receive high radiation doses, depending on the duration of exposure and the dose
rate.
The autopsy of highly radioactive cadavers should be invariably restricted to
the absolute minimum. It is essential that the staff should wear disposable gloves,
and supplementary measures for radiation protection and decontamination should be
provided in consultation with the radiation protection officer.
The embalming of cadavers constitutes an undesirable hazard and should
be avoided if possible. If the body is not autopsied and embalming is done simply by
the injection method, the contamination risk to the embalmer is small. Nevertheless,
all embalmers should wear disposable gloves. If in exceptional cases it is considered
important that the cadaver should be embalmed, the process should be permitted
only if residual activities do not exceed a value specified by the Regulatory Authority.
The embalmers must be supervised by the RPO or designate.
5.
TRANSPORTATION
Transportation of radionuclides is performed both inside and outside the hospital.
Inside the hospital the transportation includes distribution of the radioactive sources
from the storage area to the department where it will be used. Such transport should
be limited as far as possible by department design. The transport that takes place
should be performed according to optimized radiation protection conditions as given
by local rules.
The transportation of radioactive sources to and from a hospital should follow
the internationally accepted recommendations given in IAEA safety standards:
`Regulations for the Safe Transport of Radioactive Material'. These
recommendations include basic rules for the transport itself and regulations of the
shape and labeling of the packages
In general the package is built in several parts.. It should be mechanically
safe and reduce the effect of potential fire and water damage.
The package should be labeled with a sign. There are three different labels: IWhite, II-Yellow and III-Yellow. The label gives some indication of the dose rate, D, at
the surface of the package:
I- White
D  0.005 mSv/h
II- Yellow 0.005<D  0.5 mSv/h
III-Yellow 0.5<D  2 mSv/h
A more exact figure of the radiation around the package is given by the transport
index which is the maximum dose rate (mSv/h) at a distance 1 m from the surface of
the package multiplied by 100.
6.
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).
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PART 12. PUBLIC EXPOSURE
3.
INTERNATIONAL ATOMIC ENERGY AGENCY. Regulations for the Safe
Transport of Radioactive Material. Safety Series No. ST-1, IAEA, Vienna
(1996).
4.
WORLD HEALTH ORGANIZATION and INTERNATIONAL ATOMIC
ENERGY AGENCY. Manual on Radiation Protection in Hospital and General
Practice. Vol. 4. Nuclear medicine (in press)
5.
PAN AMERICAN HEALTH ORGANIZATION. Organization, development,
quality control, and radiation protection in radiology services. PAHO
Washington D.C., (1997).
6.
HARDING L.K. et al. The radiation dose to accompanying nurses, relatives
and other patients in a nuclear medicine department waiting room. Nuclear
Medicine Communications, 1990, 11: 17 -22.
7.
HARDING L.K. et al. Radiation doses to those accompanying nuclear
medicine department patients: A waiting room survey (EANM Task Group
Explaining Risks). European Journal of Nuclear Medicine, 1994, 21: 12231226.
8.
O'DOHERTY M.J. et al. Radiation dose rates from adult patients receiving 131I
therapy for thyrotoxicosis. Nuclear Medicine Communications 1993, 14: 160168.
9.
SHIELDS RA.for the Radiation Protection Committee of the British Institute of
Radiology. "The Guidance Notes Revisited: Advice to patients leaving
hospital after diagnostic nuclear medicine." (editorial). British Journal of
Radiology, 1991, 64: 567-568.
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