Methods of Safe Handling of Radionuclides and Pertaining Rules and Regulations
Any use of radionuclides is fraught with the danger of inadvertently exposing an individual to radiation, and therefore to its attendant hazards. This is especially true in the nuclear medicine laboratory where large amounts of unsealed radioactive sources are routinely handled. Each time a generator is milked, a radiopharmaceutical dose is drawn or injected, or a scan is performed on a patient, there is the possibility of exposure and contamination to the user (technician, physicist, or physician) and to the environment. Keeping this in mind, this chapter has two broad objectives: to describe the principles for minimizing radiation exposure, provide some practical ways in which an individual user can minimize exposure to oneself and fellow workers, and reduce contamination of the environment, and to give an overview of the extent and the scope of the pertinent rules and regulations that govern the use of radionuclides in nuclear medicine. For detailed descriptions, reading of the actual and most recent documents (the rules and regulations in this area are subject to periodic changes) is recommended.
Principles of Reducing Exposure from External Sources
Exposure. Hazard from external radiation sources is measured in terms of exposure. Exposure is a measure of the ability of radiation to produce ionization in air. The unit of exposure is the roentgen (R), that level of radiation which produces an ionization of 2.58 × 10-4 coulomb/kg in air. A milliroentgen, mR, is one-thousandth in a roentgen. The SI units of exposure are simply coulomb/kg. Therefore, 1 coulomb/kg = 3,876 R.
If the exposure is known at a given point, one can calculate the absorbed dose to a person at that point by multiplying the exposure by a term known as f factor. For muscle and soft tissue, f factor is close to unity. Therefore, for purposes in nuclear medicine, we may assume that exposure is more or less equivalent to the absorbed dose, which is equal to dose equivalent, that is, 1 R (1/3,876 coulomb/kg) = 1 rad (0.01 Gy) = 1 rem (0.01 Sv).
The principle of reducing exposure to radiation from sources outside the body (x- and γ-ray-emitting radionuclides only) can be summed up in three equally important words: (1) time, (2) distance, and (3) shielding.
Time: Because the total radiation dose, whether to the total body or part of the body, is directly proportional to the time of exposure, it is extremely important to spend as little time as possible near x- or γ-ray-emitting radionuclide sources. This requires forethought and precaution by the user. For example, when eluting a 99Mo generator, one should not stand near it while the elution is in progress. When injecting a dose to a patient, one
should locate the vein first and then take the syringe containing the radionuclide dose out of the leaded syringe carrier. However, rushing through a procedure is definitely not suggested here; if one has to repeat a procedure because of haste, total exposure time will be doubled.
Distance: Radiation dose to the body from a small external source varies inversely with the square of the distance of the source from the body (i.e., if we increase the distance from a source of radiation by a factor of 2, the radiation dose will drop by a factor of 4). Therefore, one should use a cart with a long handle when carrying very hot (>50 mCi) radionuclide sources emitting high-energy γ-rays (>300 keV) from one place to another, even though these sources are shielded. When radionuclide sources that are moderate in activity (≈10 mCi) are hand-carried, the source should be held away from the body.
In drawing a dose for injection, use a syringe large enough, so that it is no more than half full when the desired volume is added and handle it from the unfilled area. If it is necessary to work with hot sources for long periods, then one should seriously consider the use of various remote-control tools available for picking up a radioactive vial or pipetting a radioactive solution.
Shielding γ- and x-rays can be effectively shielded using thick containers, bricks, or partitions made of lead. Mirrors should be used for viewing behind lead partitions. The amount of actual shielding needed to reduce exposure to a minimal level depends on the amount of radioactivity to be shielded and the energy of the γ-rays. Use of a syringe lead shield to reduce exposure during radiopharmaceutical injection is essential in view of the “as low as reasonably acceptable” (ALARA) principle. However, lead aprons worn in diagnostic radiology are not very effective in reducing exposure in nuclear medicine and, therefore, are not recommended.
Calculation of Exposure from External Sources
The three variables, time, distance, and shielding, can be combined in a single formula that gives the exposure from a small radioactive source:
Here E is the exposure in roentgen R, n is the number of millicuries in the source, d is the distance (cm) of the point (at which exposure is desired) from the source, t is the time of exposure (hour), µ (linear) is the linear attenuation coefficient of the shielding material (cm-1), and x is the thickness of the shielding material (cm). is the exposure rate constant of the radionuclide, and its units are R × cm2/(mCi × h). The higher the value of of a radionuclide, the more radiation risk it produces. For 99mTc, its value is 0.60, and for 18F, its value is 5.1 R × cm2/mCi × h. As a result, for unshielded sources, 18F exposes a person 8.5 times more than 99mTc for the same amount of radioactivity.
With the increasing use of 18F and other positron emission tomography (PET) radiopharmaceuticals, this is a potentially serious source of radiation exposure to nuclear medicine personnel. It is more so because the γ-ray energy is 511 keV, which is quite high for shielding to be effective (half value layer of lead for 99mTc is 0.3 mm and for 18F is 3.0 mm. Thus, 3-mm-thick lead will reduce 99mTc exposure by a factor of 1000, but it will reduce 18F only by a factor of 2), particularly during the administration of injection and when it is in the patient. Short half-life of 18F does help in this regard. For other radionuclides of interest to nuclear medicine, values of #x393; are given in Appendix A.
(1) Calculate the exposure in 1 minute to the tips of the fingers from a syringe, which contains 15 mCi of 99mTc radioactivity, held by the tips of the fingers (assume the distance of the radioactivity from the tips of the fingers is 3 cm).
In this case,
Substituting these values in equation (18.1),
(2) Calculate the same exposure as in example 1 except this time the syringe is shielded by 1-mm-thick lead [µ (linear) = 25 cm-1]. In this case, x = 1 mm = 0.1 cm and µ (linear) = 25 cm-1.
All other factors are the same.
Therefore, using equation (18.1), we get
Thus, using the shielded syringe reduces the exposure to fingers by a factor of 12 compared to that for an unshielded syringe in example 1.
(3) A patient has been injected with 15 mCi of 99mTc radioactivity. Calculate the exposure to a technician who, on average, takes 30 minutes to perform the scan and stays at about 1 m away from the patient during this time. (Assume the radioactivity is localized in a small volume in the patient and that there is no attenuation in the patient.) In this case,
d = 1 m = 100 cm
t = 30 minutes = 0.5 hours
x = 0 and µ (linear) = 0
Using equation (18.1),
If the attenuation in the patient is also taken into account, the exposure will be further reduced, probably by a factor of 4.
(4) Same as example 3 but for 15 mCi of 18FDG.
These illustrative examples show the extent of the possible exposure in nuclear medicine. The other and very important source of exposure is the radionuclide generator.
Avoiding Internal Contamination
Internal contamination by a radionuclide is possible by three routes: penetration through skin, ingestion, and inhalation. To avoid ingestion or penetration through the skin, the following steps should be taken.
Wear coveralls or a laboratory coat and disposable gloves each time you handle a radioactive material. Remember that the gloves, once you handle a radioactive material, become contaminated. Because they are the easiest source of spread of contamination in the laboratory, discard them immediately after handling radioactive material. When handling highly radioactive material, it is wise to wear two pairs of gloves.
Do not eat, drink, or smoke in the radionuclide laboratory or pipette radioactive solutions by mouth. Before eating, when
you have been handling radioactive compounds, wash your hands thoroughly.
Keep the area neat in which radionuclides are handled. Use a tray with absorbent liners on the bench to limit the spread of radioactive material in case of an accident while working with unsealed radioactive sources.
Because the bed sheets, pillows, and stretchers used when scanning patients may be contaminated as a result of a patient’s saliva, blood, or urine, beware of this route of personal contamination.