PET is an extension of nuclear medicine. That is, the receipt, assay, administration, and imaging of unsealed radioactive sources are the core components of a nuclear medicine technologist’s profession. Surrounding these components is knowledge of nuclear medicine principles of radiation biology, radiation safety, regulatory requirements, and instrumentation for the use of radioactive materials in medical imaging, as well as knowledge of pathology and physiology in the biodistribution of those materials. This same knowledge must be employed in PET to achieve the best possible study for the patient while minimizing exposures to the patient, technologist, and general public. Considering that the photon energy associated with all PET radionuclides and radiopharmaceuticals is much higher than that used in general nuclear medicine, the radiation-safety techniques learned by those who have been formally educated in nuclear medicine technology must be closely adhered to in order to keep exposures to a minimum.
Why Radiation Safety?
Dating to the early days of the application of radiation for medical uses, it has long been known that effects are associated with exposure to high levels of radiation. However, to this day, there is much discussion and debate surrounding the effects of exposure to low levels of radiation. Based on the long latent periods associated with low levels of exposure and the onset of disease, it is very difficult to make direct cause-and-effect relationships. There-fore, statistical methods are used to estimate the risk of exposure. The primary latent disease associated with radiation exposure is cancer.
As an example, one estimate of risk of exposure at low doses is 0.0005 per rem.1 This means that if a population of 1,000,000 people were each exposed to a total body dose of 2 rem, the expected excess number of cancer deaths in that group would be 1,000. To put this exposure in perspective, a typical ALARA I (as low as reasonably achievable) limit for whole-body exposure in a medical facility is 125 mrem per quarter, meaning that the average medical radiation worker does not exceed 0.5 rem in 1 year. This is not even twice the average background radiation exposure of about 300 mrem per year in the United States.1 (The typical ALARA II level for total effective dose equivalent is 375 mrem per quarter. The US Nuclear Regulatory Commission’s licensing guide 10.8 provides guidance for establishing ALARA I and II levels for a facility.)
For comparison purposes, normal risks of illness are based on risky behavior or due to other environmental factors or causes, or combinations of these. For example, a woman has a one in nine chance of developing breast cancer in her lifetime, and one third of those women will die from the disease.2 A man has a one in six chance of developing prostate cancer in his lifetime.3 For all cancers, it is estimated that just shy of 1.4 million new cancer cases will be diagnosed among Americans in 2006.4 With recent news reports that the US population will reach 300 million this year, that calculates to an incidence rate of approximately 0.00467, or about 47 people in 10,000 who will be newly diagnosed with cancer this year. As far as risky behavior, smoking a pack of cigarettes per day is estimated to reduce a man’s life span by an average of more than 6 years. By comparison, exposure to one rem per year for 40 years is estimated to result in a life-span reduction of about 40 days.5
It is generally accepted that there is some small risk from low amounts of radiation exposure. To minimize the risks, procedures and precautions are mandated by regulations and license conditions to keep exposures low. The ALARA philosophy recognizes these risks but also recognizes that there are benefits from radiation exposure. The benefits include the ability to diagnose the illness of a patient or the ability of the technologist to earn a living. Restated, the ALARA limit seeks to minimize the cost/benefit ratio by keeping the exposure as low as is necessary and reasonable to the patient, technologist, and general public, to achieve the desired outcome.
To keep our exposures ALARA, we use practical methods of radiation safety. Everyone formally educated as a nuclear medicine technologist or radiographer learns about time, distance, and shielding such that it becomes seared into our brains. In addition, in nuclear medicine, we are taught contamination control, as virtually all of our procedures involve the use of unsealed radioactive sources. Although the basic concepts are the same, employing time, distance, and shielding is different in nuclear medicine and radiography. In radiography, it is a much more direct application of the concept. When the x-ray tube is on, it is emitting radiation. During this time, we stay out of the room or wear a lead apron to protect the parts of the body facing the exposure. In nuclear medicine, the threat is not as clear cut. We do take great precautions to store and handle radioactive materials with shielding until the material is administered to the patient. However, once that material is in the patient, the patient becomes the radioactive source. At this point, shielding might not be practical, and lead aprons are of little value, especially with the relatively high-energy photons of PET.
Compounding this issue is the fact that the patient who has been administered radioactive materials seems no different than before the administration, until the patient is near a radiation detector. Hence, it can be easy to forget practical methods of radiation safety around these patients. It is well-established that the majority of a nuclear medicine technologists’ whole-body radiation exposure is from exposure to our patients after radiopharmaceutical administration.
Many practical suggestions can be offered to highlight the appropriate use of good radiation-safety techniques.
1) Use Appropriate Shielding
The shielding used in PET must be much more substantial than that used in general nuclear medicine. The half value layer (HVL) describes the thickness of a material required to reduce the exposure rate from a source by half. The HVL of lead for the 140 keV photons of technetium 99m (99mTc) is about 0.26 mm. For the 511 keV annihilation photons associated with a PET radionuclide, such as fluorine 18 (18F), the HVL of lead increases to 4.0 mm.5 In other words, given an equal exposure rate caused by two sources, one 99mTc and one 18F, about 15 times the thickness of lead is required to reduce the exposure from the fluorine compared to the technetium.
This has two significant implications in PET. First, the shielding used in general nuclear medicine is not sufficient for use in PET. The typical syringe shield used in general nuclear medicine is designed primarily for use with 99mTc. Commercially available devices for use in general nuclear medicine have a 1.5-mm tungsten wall and decrease the exposure from 99mTc by about 94%. PET syringe shields are available in thicknesses of 8.64 mm and 14 mm of tungsten. The 14-mm shield decreases the exposure from a PET radionuclide by about 97%.6
CT Radiation Safety
A brief discussion of CT radiation safety is necessary. The basics of occupational radiation safety in CT primarily involves staying out of the room when the x-ray tube is energized. This includes the tube warm-up period. If personnel must be in the scanner room during operation of the CT scanner, they must wear a lead apron.
A larger issue with the fusion of PET and CT is the qualifications of the person operating the machine, or each portion of the machine. The rules surrounding who can do what vary dramatically from state to state. An in-depth discussion of the debate surrounding this issue is beyond the scope of this article. However, patient radiation safety dictates that kV and mAs be adjusted based on patient size to minimize patient exposure from this portion of the exam.
Depending on the technique used, the radiation exposure to the patient from the CT portion could be several times that which the patient receives from the radioactive material administered for the PET portion of the exam. It is generally accepted that the adjustment of the kV and mAs falls within the scope of practice of a radiographer and outside of the scope of practice of a nuclear medicine technologist, based on current definitions of the professions.
The second important implication is that, due to the thicker shielding needed, the shielding will be much heavier. A standard shipping container for a single unit dose PET radiopharmaceutical can weigh about 70 pounds. Although not a radiological hazard, the significant weight means that consideration must be given to the technologists handling these doses so that they are not injured while moving these containers.
2) Follow Package-Receipt Protocols
Regulations and/or license conditions generally require that a radioactive package labeled as a white I, yellow II, or yellow III be surveyed for ambient exposure rate and removable contamination upon receipt. The survey for removable contamination, or wipe test, must be conducted over 300 cm2 on each package and may not exceed 22 dpm/cm2, or a total of 6600 dpm over the 300 cm2. The normal scenario is to wipe-test each package individually; however, there is no requirement to do so. In the interest of limiting time near the PET shipping container—and because several shipping containers will likely be received, as each usually holds only one dosage—it is recommended that a single wipe test be used for all of the packages. This would still need to be completed over 300 cm2 on each package. If contamination exceeding the limit was detected, there is the risk of having spread the contamination to other packages and each package would have to be individually retested. However, finding a contaminated package is a rare event.
After completing the receipt procedure, the packages should be stored in a remote area of the hot lab to avoid exposure to the technologist and the equipment, to the extent possible and practical. Storing the packages in close proximity to the dose calibrator and/or well counter may result in erroneous readings. Because information on injected dosage is part of the equation for determination of standard uptake value (SUV), erroneous dose-calibrator readings will cause erroneous SUVs.
3) Assay Radiopharmaceuticals Immediately Prior to Injection
To minimize the handling of the radiopharmaceutical dosage, it should be assayed immediately prior to administration and placed directly into the injection device. This also allows the dosage to be at the lowest possible activity when it is handled. In addition, full advantage of the available shielding should be taken during this procedure. For example, to assay the dosage, it also can be held in the shipping pig while being moved to the dose-calibrator chamber and transferred with forceps. An even better scenario is to have the chamber sunken into the counter behind the L-block so that the dosage never has to be raised above the shielding. In the case of the sunken dose-calibrator chamber, auxiliary chamber shielding or a shielded cabinet will be necessary to avoid exposure to the technologist’s reproductive organs.
4) Handle as Little as Possible
In addition, it is recommended that dosages not be routinely adjusted by the technologist to keep extremity exposure to a minimum. An exception to this recommendation would be in the case of reducing a dosage for a child. To avoid the need for adjusting dosages, a prescribed dosage range is recommended. This is based on the applicable rule in most, if not all, places that the technologist may vary from the prescribed dosage or dosage range only by 10% without prior authorization from an authorized user. With the short half-lives of the PET radionuclides used today, if the dosage is assayed much before or after the calibration time, it can easily be outside of 10% of a prescribed dosage. An example of such a range is 10 to 20 mCi for fluorine 18-labeled deoxyglucose (FDG-18).
5) Talk First, Administer Radiopharmaceuticals Later
|The round storage container from Fluke Biomedical is designed expressly for use with PET radionuclides.|
It is recommended that obtaining history and all other verbal interaction with the patient occur before administration of the radiopharmaceutical. This allows the technologist to assess whether the test is appropriate and, if so, if the correct patient preparation has been followed to allow for a worthwhile study. It also saves the technologist from the exposure to the injected patient.
6) Place a Catheter or Butterfly
Due to the bulk of a PET syringe shield, it would be very difficult to administer the material using a straight-stick technique. The intravenous (IV) line also allows for establishment of IV access so that the material can be quickly administered with less chance for infiltration (extravasation into the soft tissue around the injection site). The IV line must be flushed with saline after dosage injection to ensure complete dosage administration and to minimize residual activity in the IV line. Infiltration causes the patient unnecessary radiation exposure if it causes a suboptimal study that needs to be repeated, not to mention that it wastes an expensive dosage that cannot be replaced easily. It also should be noted that SUVs are worthless when the dose is infiltrated.
|The RADSAFE syringe shield from Fluke Biomedical is designed expressly for use with PET radionuclides.|
Several remote injection devices are on the market. If such a device is to be employed, it should be determined that the time it takes to load the syringe into the device does not eliminate any benefits of the shielding provided. At least one radiopharmaceutical vendor, Fluke Biomedical, Cleveland, has a delivery pig with removable top and bottom sections that allows the end-user access to both ends of the syringe without removing the syringe from the shielding. Another perfectly acceptable alternative is the use of an appropriate PET syringe shield. In any case, where a device is used such that the ends of the syringe are not shielded, it should be used in a way that the syringe/device is parallel to the technologist so that the technologist achieves the maximum benefit of the shielding. Also, syringe shields are usually cylindrical, so care must be taken to avoid having it fall off the injection platform.
7) Restrict Access to Holding Areas
If at all possible, family members should not be allowed to stay with the injected patient. Not only does this avoid unnecessary exposure to the family member, but—in the case of FDG-18—it avoids head- and neck-muscle uptake that could result from the patient talking with the family member during the uptake phase.
8) Shield the Waste-Storage Area
As with any nuclear medicine procedure, radioactive waste will be generated. The good news is that it often can be held for a day or less before being discarded as normal or biohazardous trash—all other considerations of the rules of decay-in-storage notwithstanding. However, for a time, the radioactive waste can be quite “hot.” Therefore, appropriately shielded waste-storage areas must be available. In addition, to the extent possible, the waste-storage area should be away from the dose-preparation area and the measurement equipment for the reasons mentioned previously regarding storage of radioactive packages.
9) Escort the Patient—But Be Quick
Exposure from patients is the primary source of whole-body radiation exposure for nuclear medicine technologists, so escorting the patient to and from the restroom and scanner after radiopharmaceutical administration is an area where technologists can dramatically impact their exposures. Especially in cases where the patient-holding room, restroom, and scanner are significant distances from one another, as is sometimes the case in the mobile PET environment, consideration should be given to transporting the patient by wheelchair. The current state of PET is that the typical patient is an oncology patient. Such patients are often weak or in pain and require assistance and a slow pace. The use of a wheelchair to transport the patient not only speeds the process and increases the distance between the technologist and patient, thereby reducing the technologist’s exposure, but it also is easier on the patient.
10) Consider a Department Redesign
Many of the situations in the aforementioned examples can be mitigated through thoughtful department design. Radiation-safety issues are addressed more easily during this time. Designing shielded patient-uptake/holding rooms; locating the holding rooms, restroom, and scanner room in close proximity; and designing the hot lab, including an adequate shielding designed for the 511 keV photons of PET, are just some of the important issues to consider.
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In conclusion, it is clear that knowledge and application of good radiation-safety practices and regulations associated with the use of unsealed radioactive sources is critical in PET. Only by adheing to these practices can we reasonably expect to keep exposures to workers, patients, and the general public to a minimum. Just as CT parameters should be adjusted only by a professional specifically educated to do so, only those professionals who are specifically educated in the use of unsealed radioactive materials—such as a certified nuclear medicine technologist—should be employed in the use of those materials associated with PET.
Leroy H. Stecker, III, MBA, RT(N), CNMT, is the program director for the Nuclear Medicine Technology Program at St Vincent’s Medical Center (Jacksonville, Fla) and has been a nuclear medicine technologist for 20 years.
Safety During Pregnancy
The declared pregnant radiation worker was first addressed by the US Nuclear Regulatory Commission (NRC of Washington, DC) in the mid-1990s. It is now incorporated into the regulations of most, if not all, states. To paraphrase, the rule states that a radiation worker who becomes pregnant may voluntarily declare to her employer that she is pregnant. On doing so, the employer is responsible for ensuring that the worker’s fetus does not exceed a limit of 500 mrem from the estimated date of conception to the end of the pregnancy. Typically, a dosimeter is supplied to the woman, which is worn at waist level and under any protective garment to estimate fetal dose.
In general nuclear medicine, a typical ALARA I limit for whole-body exposure is 125 mrem per quarter. This means that we do not normally expect a technologist working in that area and using appropriate radiation-safety practices to exceed 375 mrem in a 9-month period. Therefore, although the discussion of declared pregnancy definitely applies in that situation, it is largely academic. However, the PET technologist most likely will exceed 500 mrem during the 9-month period. An estimate of exposure to any given technologist is not particularly useful, as too many variables are involved. Such variables include department layout, staffing, number of patients per day, available shielding, and the individual’s adherence to good radiation-safety practices. However, it is clear that the declared pregnant PET technologist may well need to be removed from the exposure associated with the PET department before the end of the pregnancy.
- Contemporary Physics Education Project (CPEP) and the Nuclear Science Division. Guide to the Nuclear Science Wall Chart, 2nd ed, 2001. Available at: www.lbl.gov/abc/wallchart/guide.html. Accessed July 18, 2006.
- Cotran SC, Kumar V, Collins T, eds. Robbins Pathologic Basis for Disease, 6th ed. Philadelphia, Pa: WB Saunders Co; 1999.
- American Cancer Society. Overview: prostate cancer. Available at: www.cancer.org/docroot/CRI/CRI_2_1x.asp?dt=36. Accessed July 18, 2006.
- American Cancer Society. Cancer facts & figures 2006. Available at: www.cancer.org/downloads/STT/CAFF2006PWSecured.pdf. Accessed July 18, 2006.
- Early PJ, Sodee DB. Principles and Practice of Nuclear Medicine, 2nd ed. St Louis, Mo: Mosby-Year Book; 1995.
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