Therapeutic Nuclear Medicine - General Overview

1. Overview of therapy with unsealed radionuclides
2. Properties of some radionuclides used in therapeutic nuclear medicine
3. Radiation safety considerations - general
4. Decision to hospitalize or discharge a patient

1. Overview of therapy with unsealed radionuclides

The basis of radionuclide therapy is placement of the radionuclide in intimate contact with the target tissue. Particularly if short-range particle emitters are used, the absorbed dose to the target is very high compared to that to non-target tissues.

Radionuclides are gaining increasing importance by providing palliative and curative treatment in a number of malignant diseases. Most radionuclides used in radionuclide therapy radiate beta particles which have a low range of tissue penetration. A few emit auger electrons and alpha particles, and several also emit gamma rays and X rays during their decay. The most successful metabolic radionuclide therapy uses iodine-131 as the nuclide for the treatment of benign hyperthyroid conditions and thyroid carcinoma.

Pure beta particle emitters, with their lower penetration ability, deposit all their energy within the patient, ideally at the target site. From a radiation safety perspective, there is significant radiation limited to the patient, with little exposure to the public. Concern with radiation safety is, therefore, related to the handling of excreta and body fluids when we deal only with beta emitters, such as strontium-89 or rhenium-188. However, when radionuclides that emit gamma rays - such as iodine-131 - or bremsstrahlung X rays are used, radiation safety must consider the exposure of the public to the gamma emissions.

Systemic intravenous administration of radionuclides is common but there are several other methods of administration, e.g. radioactive material may be instilled into a body cavity.

The clinical use of phosporous-32 and the radionuclide palliation of painful bone metastases, highlight the changing roles of some radionuclide therapy. Phosphorous-32 has proved to a be good therapeutic modality in myeloproliferative diseases, but the secondary induction rate of leukaemia is not completely negligible and thus this therapy is being largely replaced by chemotherapy, which appears to have fewer adverse side effects. Conversely, palliative therapy of painful bone metastases, no longer responding to other therapy, is now widely practiced using strontium-89 and the recently introduced rhenium-188 and samarium-153. Up to 70% of patients have a remission from pain, permitting them to reduce their intake of strong analgesics, which can last for several months.

Some of the more common radionuclides used for therapy and their main radiation emissions are found in the following table.

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2. Properties of some radionuclides used in nuclear medicine therapy

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3. Radiation safety considerations - general

In some cases, the patient is treated in hospital and allowed to go home immediately, but in other cases, the patient must be hospitalized until such time - considering the patient's family and the general public - as it is safe to allow discharge.

There are two radiation safety considerations that must be addressed: external radiation, and contamination by excretion of the radionuclide. The external radiation is simply related to the radionuclide used, and its emissions and half-life. Excretion however brings the possibility of contamination of the patient’s environment, and possible ingestion by other persons. These are considered separately for each therapy radiopharmaceutical in other modules.

Should the patient die following discharge, the problem of safe disposal of the body must be considered.

External radiation

The therapy patient is also a source of radiation. If the radionuclide used is a low energy beta emitter, then this radiation might not even be detectable, or be only a low level. On the other hand, if a gamma emitter is of medium energy, then the radiation levels in the vicinity of the patient can be significant. At short distances, the inverse square law does not hold because the source is often distributed over a large area, so distance from the patient is not as effective as if they were a point source. However, beyond about three metres distance, the inverse square law may be used.

While the photon radiation exposure to family and caregivers is not significant in terms of future cancer induction, it can and thus should be controlled and minimized. In particular, children and pregnant women should be protected.

Excretion

The radiopharmaceuticals used may have a number of fates:

• Total and permanent retention within the body. This is normally confined to particles which are trapped within a particular organ or system
• Partial trapping and retention of particles, with potential excretion of unbound radionuclide.
• Partial trapping and retention of ionic compounds, with excretion of the remainder.

The excretory pathways include:

• Urine
• Faeces
• Saliva
• Sweat
• Lacrimal fluid
• Breast milk

Each pathway has different safety issues, and all can lead to contamination. In particular, the clearance rate of the radionuclide from the patient’s body can vary greatly, not only between radiopharmaceuticals, but also for the same radiopharmaceutical. The main route of excretion is generally through the kidneys leading to high concentrations of radionuclides in urine and discharge of radionuclides to the sanitary sewer. Some data are found in the table below.

Cross-contamination from patients to other persons is, however, generally less of a safety hazard than external radiation exposure. The likelihood of a significant intake by contamination is small, and poses an extremely low cancer risk. Possibly the greatest likelihood is of hypothyroidism due to ingestion of significant quantities of iodine-131.

Proportion of administered activity (until total decay) which is discharged to the sanitary sewer for some therapeutic radionuclides

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4. Decision to hospitalize or discharge a patient

The IAEA Basic Safety Standards (BSS) and many national authorities apply limits to the retained activity level at which a patient may be released. These limits are based, in theory, on the estimated exposure to any member of the public remaining below a certain effective dose. This is either 1mSv (the dose limit for a member of the public), or 5 mSv, on the basis that - under ICRP and BSS recommendations - caregivers may receive higher doses than the public dose limit. Any restrictions should be based on the sensitive group of infants and children. The current BSS give a guidance level for release of Iodine-131 patients (treated by any form of therapy), which is 1100 MBq. Other bodies such as the National bodies in the Australia, Sweden and the United States, have their own guidelines. Most other guidelines are specific for Iodine-131 only.

The release limits vary widely, however, and take into account only the retained activity and no other factors such as self-absorption in the patient. The assumptions used in generating the limits probably significantly overestimate potential doses to caregivers and the public. The ICRP recommends that the decision should be determined on an individual basis, taking into account patients' pattern of contact with other people, their age and that of persons in the home environment, patients’ wishes, and local social and infrastructure issues. The cost of hospitalization, and radiation exposure of hospital staff should also be considered (summary of issues to be considered).

Estimation of exposure from external radiation is based not only on clearance from the body, but also on distance and exposure time. A profile of the time spent at certain distances from the patient can be developed for the spouse, and for other family members, which can then be used to estimate cumulative dose. Even these methods can significantly overestimate risk. If patients and caregivers follow simple radiation protection precautions, such as outlined later in this guideline, effective doses would rarely approach, let alone exceed, the 5 mSv level.

Experience shows that the occupational exposure to staff as a result of patients being held in hospital after therapy is generally very low or negligible. The major advantage of holding the patient is the control of the environment. The decision to release the patient is then based on the risk to others from this loss of control, balanced by the advantages to the patient and family. The BSS (para. II-9) states that “..the dose of any comforter or visitor of patients shall be constrained so that it is unlikely that this or her dose will exceed 5 mSv during the period of the patient’s diagnostic examination or treatment. The effective dose to children visiting patients who have ingested radioactive materials should be similarly constrained to less than 1 mSv.”

Release criteria for various radionuclides

Other iodine-131 release criteria

Issues to be considered in release of therapy patients

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