Preventing unnecessary exposure in diagnostic nuclear medicine

Frequently asked questions by the health professionals

» What are the radiation risks associated with nuclear medicine examinations?
» Can I determine the patient exposure given in an examination?
» Can a patient who is breastfeeding a baby continue to do so?
» Can patient protection be considered in the selection of radiopharmaceuticals?
» Should patient protection be considered in the preparation and administration of radiopharmaceuticals?
» What principles should be used when prescribing the activity to administer?
» Are there guidance levels upper limits of administered activities?
» How do I know that I am administering the correct activity?
» Can I reduce the effective dose to the patient after administration of the radiopharmaceutical?

» What are the radiation risks associated with nuclear medicine examinations?

Nuclear medicine examinations are quite safe when it comes to the activities of radiopharmaceuticals administered for diagnostic purposes. In cases of extravascular injection of a high activity in a small volume of a radiopharmaceutical, which may have a very slow outflow from the injection site, the absorbed dose at this location can be high enough to cause a deterministic effect. Extravascular injection could be considered an accidental exposure because it is substantially different from the intended exposure. 

» Can I determine the patient exposure given in an examination?

Yes and no. Yes as the absorbed dose and organ doses  can be calculated, and no because there is no method to directly measure the dose to an organ from a radiopharmaceutical. A schema for calculation of absorbed doses has been developed by The Medical Internal Radiation Dose Committee (MIRD) and this methodology has been widely adopted. International Commission on Radiation Protection (ICRP) has introduced biokinetic models and data. This has resulted in estimation of the effective dose  for a large number of radiopharmaceuticals. The figures are given in mSv/MBq meaning that an estimation of the effective dose to a single patient can be made simply by multiplying with the administered activity. Note that variations from patient to patient can be very large.

The answer to the question is that a rough estimation of the effective dose to the patient can be made. For calculating absorbed dose and organ doses for a single patient, one requires knowledge of the emissions of the radionuclide, the activity administered, the activity in the specific organ and in all other organs, the size and shape of the organ, the kinetic properties of the radiopharmaceutical and its quality. These factors are not readily available for the single patient.

» Can a patient who is breastfeeding a baby continue to do so?

It depends on the radiopharmaceutical and its activity. Before administering a radiopharmaceutical to a woman who might be breastfeeding, an inquiry should be made and consideration should be given as to whether:

• the test could reasonably be delayed until after the mother has ceased breastfeeding; 
• the most appropriate choice of radiopharmaceutical has been made bearing in mind the secretion of activity in breast milk.

In order to minimize potential irradiation of a breastfed child, advisory notices should be posted within the nuclear medicine department asking patients to inform the staff if they are breastfeeding.

The following general guidelines from the ICRP Publication 84 - Pregnancy and Medical Radiation can be given for recommended periods of interruption for breastfeeding as examples of good practice:

• three weeks after 125I radiopharmaceuticals except labelled hippurate, and after 22Na, 67Ga and 201Tl;
• 12 hours after iodine-labelled hippurates and 99mTc compounds except labelled red blood cells, -phosphonates and -DTPA;
• four hours after 99mTc compounds labelled red blood cells, -phosphonates and -DTPA;
• breastfeeding is to be totally avoided after 131I administration for therapeutic purpose.

» Can patient protection be considered in the selection of radiopharmaceuticals?

Yes.

If more than one radiopharmaceutical can be used for a procedure, consideration should be given to the physical, chemical and biological properties of each radiopharmaceutical so as to minimize the organ doses to the relevant tissues and other risks to the patient while at the same time providing the desired diagnostic information. The user also has the obligation to use only licensed producers of radiopharmaceuticals, who can guarantee that the production is according to good manufacturing practice and complies with relevant international standards.

» Should patient protection be considered in the preparation and administration of radiopharmaceuticals?

Yes. 

Giving the correct radiopharmaceutical and correct activity to the correct patient starts with the preparation procedures. Locally designed working procedures should ensure that the risk of mixing different preparations is reduced to a minimum. Each preparation should be labelled indicating the radiopharmaceutical, the volume of the preparation and its activity at a given reference time. Check the preparation before dispensing the radiopharmaceutical. Measure the activity and write a label stating the radiopharmaceutical, activity and reference time.

The route of administration should also be part of prescription and labeling. A single dose should also indicate patient name or identification number.

The patient should be identified using the general rules in the hospital. All patients should be informed about the procedure. Female patients should be asked about pregnancy as well as breastfeeding. The request form should be checked against the label of the dispensed radiopharmaceutical to make sure that the preparation is the correct one for the requested examination.

» What principles should be used when prescribing the activity to administer?

For any given procedure, the optimum activity will depend on the body weight of the individual patient, the patient's metabolic characteristics and clinical condition, the type of equipment used, the type of study and the examination time.

For a given type of imaging equipment, the diagnostic value of the information obtained from a test will vary with the amount of administered activity. There is a threshold of administered activity below which no useful information can be expected. Above this level, the diagnostic quality will increase steeply with the increasing activity. Once an acceptable quality of the image has been reached, any further increase of the administered activity will increase only the absorbed dose and organ doses and not the value of the diagnostic information. For children, the administered activities should be reduced in proportion to body weight or any other established principle.

» Are there guidance levels upper limits of administered activities?

No. 

Guidance levels should assist in the optimization of protection by helping to avoid unnecessarily high patient exposure. The system for using guidance levels includes the estimation of patient exposure as part of the regular quality assurance programme. Guidance levels are not to be applied to individual exposures of individual patients. It is a level set for standard procedures for groups of standard sized patients or a standard phantom. Guidance levels are supplements to professional judgement and do not provide a dividing line between good and bad medicine.

The guidance levels should be based on wide-scale surveys and aim to represent the current state-of-the-art practice. They should be revised as technology improves. In lack of nationally established guidance levels, the ones from the IAEA International Basic Safety Standards can be used.

» How do I know that I am administering the correct activity?

The only reliable way is to measure the activity using an activity meter specially designed for this purpose. The IAEA International Basic Safety Standards state that unsealed sources for nuclear medicine procedures should be calibrated in terms of the activity of the radiopharmaceutical to be administered, the activity being determined and recorded at the time of administration. At least one activity meter will be required in the nuclear medicine department.

» Can I reduce the effective dose to the patient after administration of the radiopharmaceutical?

Yes. 

Substantial reduction in effective dose from radiopharmaceuticals can be readily achieved by the following simple measures:

• Hydration of the patient is an effective means to reduce the absorbed dose and organ doses from radiopharmaceuticals that are excreted by the kidneys. The patient should be encouraged to void frequently, especially immediately following the examination;
• The use of thyroid blocking agents such as potassium iodide or potassium perchlorate is a generally accepted practice when using radioactive iodine compounds, excluding thyroid related conditions.

When examining a pregnant woman using a radiopharmaceutical that is rapidly eliminated by the kidneys, the maternal urinary bladder can become a major source of irradiation to the fetus. Frequent voiding should therefore be ensured. The absorbed dose to fetal tissues can be further reduced by administering the radiopharmaceutical when the bladder is partially filled, rather than immediately after voiding.