Dauer, Lawrence T.*
Physicians, nurses, technologists, and others involved in medicine constitute the largest single group of workers occupationally exposed to manmade sources of radiation (UNSCEAR 2008). Therefore, it is appropriate that the National Council on Radiation Protection and Measurements (NCRP) include a discussion on exposed medical staff in its 49th Annual Meeting on Radiation Dose and the Impacts on Exposed Populations. This paper addresses the principles of radiation protection, the recent evolution of the understanding of the effects of radiation on the lens of the eye, and assesses exposed medical staff by evaluating the annual doses at a high-volume medical facility and commercial radiopharmacies. Finally, it discusses three current exposure potentials that represent unique challenges for radiation protection of medical staff, including fluoroscopy-guided interventions (FGI), radioactive material use in diagnostic imaging, and the expanding novel uses of radiation in medicine. The purpose of this paper is to identify challenges, available tools, and opportunities for improvement in the protection of medical staff.
FUNDAMENTAL RADIATION PROTECTION PRINCIPLES
NCRP radiation protection objectives
When delineating the purposes of radiation protection, NCRP addresses two overarching purposes (NCRP 1993). The first is to prevent the occurrence of clinically significant radiation-induced deterministic effects by adhering to dose limits that are below the apparent threshold levels. The second is to limit the risk of stochastic effects, cancer, and genetic effects to a reasonable level in relation to societal needs, values, benefits gained, and economic factors. NCRP further espouses the three principles of radiation protection (justification, optimization, and limitation) as essential elements of a framework for meeting the objectives (NCRP 1993). In this schema, justification is on the basis that the expected benefits to society exceed the overall societal cost. Optimization is a process to ensure that the total societal detriment from justifiable activities is maintained as low as reasonably achievable, economic and social factors being taken into account. Limitation is the application of individual limits to ensure that procedures of justification and optimization do not result in individuals or groups exceeding levels of acceptable risk.
ICRP radiation protection objectives
The approach of the International Commission on Radiological Protection (ICRP) differs somewhat in philosophy from the NCRP. The aim of the ICRP recommendations is to contribute an appropriate level of protection against the detrimental effects of radiation exposure without unduly limiting desirable human actions associated with such exposure, and to manage and control exposures to ionizing radiation so that deterministic effects are prevented and the risks of stochastic effects are reduced to the extent reasonably achievable (ICRP 2007). ICRP considers “detriment” to be the total harm to health as a result of exposure to a radiation source. In addition, its understanding of such detriment with regard to organs or tissues has evolved (ICRP 2012). Tissue reactions (previously called deterministic effects) are the detriments arising from noncancer effects of radiation on health. This is in recognition that some effects are not determined solely at the time of irradiation but can be modified after exposure (ICRP 2012).
ICRP reviewed recent epidemiologic evidence suggesting that there are some tissue reaction effects, particularly those with very late manifestation, where threshold doses (i.e., the dose resulting in only 1% incidence of special tissue or organ reactions) are or might be lower than previously considered, such as in circulatory disease and lens of eye damage. The ICRP emphasizes that protection should be optimized not only for whole-body exposures but also for exposures to specific tissues, particularly the lens of the eye, the heart, and the cerebrovascular system.
Lens doses are of particular concern in medical staff exposure situations. Often, posterior subcapsular opacities are the initial manifestation of radiation damage to the lens. Cataracts are the ultimate expression of radiation damage to the lens. Latency depends on the rate at which damaged epithelial cells undergo fibrogenesis and accumulate. ICRP evaluated lens dose effects studies of atomic bomb survivors, acute radiation exposures, clinical patients (diagnostic and therapy), radiation workers (including radiologic technologists, interventional radiologists, cardiologists, and Chernobyl cleanup workers), other workers, and individuals exposed to chronic low doses (ICRP 2012) [see review of recent studies by Roy Shore in ICRP (2012) Appendix A]. Based on this evidence, ICRP suggests that the threshold for lens detriment is around 0.5 Gy for acute or protracted exposures. Therefore, for occupational exposure in planned exposure situations, ICRP now recommends an equivalent dose limit for the lens of the eye of 20 mSv y−1 averaged over defined periods of 5 y, with no single year exceeding 50 mSv (ICRP 2012). Table 1 lists and compares the most recent NCRP and ICRP occupational dose limits, as relevant for exposed medical staff.
EXPOSED MEDICAL STAFF
Medical staff exposures have increased substantially over time (UNSCEAR 2008; NCRP 2009), especially in the last decade or so. There are two fundamental reasons for such increases. First, the worldwide rate of certain diseases continues to rise. Second, technological developments have increased the beneficial and effective use of radiation and radiation-producing devices in the diagnosis and treatment of those diseases. Medical staff doses generally track upward as the number of patient cases increases.
Increase in patient doses
The World Health Organization (WHO) has noted that worldwide cancer rates continue to rise. In 2008, the WHO estimated that 7.6 million deaths (or 13% of all deaths) were the result of cancer, especially of the lung, stomach, liver, colon, breast, and cervix (IARC 2008; WHO 2013a). Cancer cases are expected to continue to rise up to ~26.4 million cases and 13.1 million deaths by 2030 (IARC 2008). Imaging and radiotherapy play important roles in cancer management. Indeed, advances of the last 10 y have shifted goals from life preservation to cure with increased quality of life.
Cardiovascular disease (CVD) rates also continue a worldwide climb. In 2008, the WHO estimates that 17.3 million deaths were due to CVD, with 7.3 million deaths due to coronary heart disease and 6.2 million deaths due to stroke (WHO 2013b). It is estimated that up to 25 million deaths due to CVD could be expected by 2030. Imaging and especially FGI procedures play important roles in CVD diagnosis and management.
The rising use of radiation in medicine is particularly evident in the United States, where the annual effective dose per capita for medical procedures rose from 0.5 mSv in 1980 to 3.0 mSv in 2006 (NCRP 2009). This can be compared to the worldwide increase from 0.3 mSv to 0.6 mSv over the same timeframe (UNSCEAR 2008). As of 2006, it was estimated that ~337 million diagnostic or interventional radiology procedures and 18 million nuclear medicine procedures were performed in the United States (Mettler et al. 2008). The worldwide estimate was as high as 3.6 billion radiation procedures. It is clear that much of this increase was due to the increased usage of computed tomography (CT), which was growing at ~10% per year at one point, reaching as high as ~80 million examinations per year in the United States, with ~10% of those cases in children. The use of CT in the emergency department continues to double approximately every 4.7 y, growing at a rate of ~16% per year (Larson et al. 2011). Nuclear medicine use has risen as well and now represents ~12% of the annual effective dose per capita in the United States in the most recent comprehensive assessment (NCRP 2009).
Increase in medical staff doses
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) identified that the worldwide collective effective dose and average annual effective dose for the exposed population had changed from 760 person-Sv and 1.4 mSv, respectively, in 1994, to 3,540 person-Sv and 1.24 mSv, respectively, in 2002 (UNSCEAR 2008). UNSCEAR identified that physicians, technicians, nurses, and others involved in medicine constituted the largest single group of workers occupationally exposed to manmade sources of radiation. While >80% of CT technologists and general radiographers did not have measurable exposures, interventional radiologists and cardiologists were the most exposed practitioners in medicine (UNSCEAR 2008). Table 2 summarizes the UNSCEAR estimates by medical staff exposure category as of 2002 and shows that in addition to those involved in FGI procedures, those involved in nuclear medicine imaging/therapy also received exposures greater than the average medical staff.
NCRP later updated estimates of medical staff annual exposures and found that these contributed the most (39%) to the U.S. occupational exposures, with ~0.75 million of 2.5 million monitored workers receiving measurable annual doses (NCRP 2009). It was estimated that the annual collective effective dose in 2006 was ~550 person-Sv with an average annual effective dose to the exposed workers of ~0.75 mSv. About 70% of this occupational dose was received in hospital settings, another 23% in other medical facilities, with the remainder in veterinary, veteran’s affairs, and dental and medical school settings (NCRP 2009).
Occupational doses in a high volume hospital setting
An evaluation of the occupational doses in a high volume hospital setting was performed based on a review of personal dosimetry records for all monitored workers in the year 2011 at Memorial Sloan-Kettering Cancer Center. Hospital operations staff monitored 3,820 individuals. The mean deep dose equivalent (DDE), lens dose equivalent (LDE), and shallow dose equivalent (SDE) for all monitored workers were 0.14, 0.20, and 0.80 mSv, respectively. It should be noted that 89% of the monitored workers received no measurable doses. The mean DDE, LDE, and SDE for those who received measurable doses were 1.5, 1.8, and 7.3 mSv, respectively. Figs. 1, 2, and 3 display the distribution of measurable doses and show that for DDE, LDE, and SDE, the distribution is skewed or characterized by an asymmetric shape with a main peak at the lowest dose region, a tail, and a few higher values.
Table 3 (DDE), Table 4 (LDE), and Table 5 (SDE) show how these doses are distributed by category of exposed medical staff and include data for six commercial radiopharmacies in the United States (data from Landauer, Inc., Illinois) for comparison. The tables include average; minimum; 25th, 50th, 75th, 95th, 99th percentiles; and maximum mSv y−1. For DDE (Table 3), those involved in nuclear medicine and FGI procedures received higher than the hospital average whole-body doses, while radiation safety personnel and nurses received less than the average. Maximum doses were <16 mSv y−1 for all hospital categories. The most exposed category of worker in the hospital was the radiopharmacist who received an average of 4.6 mSv y−1 and a maximum of 8.5 mSv y−1. Comparatively, commercial radiopharmacies had a 99th percentile dose of 22.2 mSv y−1 and a maximum dose of 47.4 mSv y−1. Nuclear medicine technologists/nurses and physicians in the hospital were the next most-exposed category, followed by the FGI physicians and research radiochemistry workers. Inpatient nurses received the lowest measurable dose with a mean of 0.4 mSv y−1.
For LDE (Table 4), FGI physicians (if not protected with lead ceiling shields or glasses) received the maximum doses to the lens, with an average of 11.1 mSv y−1, a 75th percentile of 19.3 mSv y−1, and a maximum of 36.5 mSv y−1. Some FGI technologists and nurses received lens doses of up to 19.3 mSv y−1 (if not protected). This was also true for some nuclear medicine technologists and nurses with doses as high as 19.0 mSv y−1. Assessment of protection practices and personal protective equipment use is therefore important for FGI staff as well as nuclear medicine workers. The hospital average LDE was 2.1 mSv y−1. Comparatively, while commercial radiopharmacy workers recorded 1.6 mSv y−1 average LDE, the 99th percentile lens dose was 23.5 mSv y−1, and the maximum was 70.2 mSv y−1. Inpatient nurses received the lowest average LDE with 0.4 mSv y−1.
For SDE (Table 5), commercial radiopharmacy workers received higher skin/hand/extremity doses, with an average of 50.5 mSv y−1 and a maximum of 364 mSv y−1. In the hospital, radiopharmacists similarly received the highest doses with an average of 41.8 mSv y−1 and a maximum of 168 mSv y−1. Some research radiochemists in the hospital also received maximum doses above 100 mSv y−1. The hospital average SDE was 6 mSv y−1 with radiopharmacists, research radiochemists, and FGI physicians receiving more than the hospital average, while technologists/nurses, nuclear medicine workers, brachytherapy workers, and inpatient nurses received less.
FLUOROSCOPICALLY-GUIDED INTERVENTIONAL PROCEDURES
Medical practitioners performing FGI procedures typically receive occupational exposures, and several studies have shown that they may also be exposed to a relatively high ocular dose, especially when protection tools are not used (Vano et al. 2006, 2008; Kim et al. 2008, 2012; Dauer et al. 2010; NCRP 2010). FGI staff lens doses vary by procedure, training, methodology, complexity, patient factors (e.g., size and image location), and equipment. Table 6 shows nominal estimates of unshielded LDE in terms of millisievert per procedure adapted from available literature (Vano et al. 1998, 2010; Ciraj-Bjelac et al. 2010; Dauer et al. 2010; NCRP 2010). Some procedures can result in almost 1 mSv per procedure to the lens of the primary physician without protection. It has also been shown that LDE generally correlates with patient dose, with a factor of ~4–7 μSv LDE per Gy cm2 kerma-area-product received by the patient (Dauer et al. 2010). Increased lens dose may result in increased risk of lens opacification and ultimately cataracts over a working lifetime (NCRP 2010; ICRP 2012). The use of adequate eye protection is clearly a necessity, especially for high-volume practices (Dauer et al. 2010; NCRP 2010). Leaded glasses have been shown to reduce lens doses by a factor of about three (or higher), shielded sterile drapes by a factor of ~25, and suspended ceiling shields by a factor of roughly over 100 (Thornton et al. 2010).
Protection controls for FGI staff are outlined well in NCRP Report No. 168 (NCRP 2010), include engineering, safe work practices, administrative controls, and personal protection equipment. Fluoroscopy suite engineering includes optimization of equipment, structural shielding, and equipment shielding (including table skirts and ceiling suspended shields). Safe work practices can be codified in standard operating procedures as well as the use of safety wisdom practices, such as the “10 pearls” for FGI (IAEA 2013). Personal protective equipment considerations include aprons, collar shields, leaded glasses, and ceiling-suspended or rolling shields. Although numerous advances in fluoroscopic equipment design and shielding approaches (both body and eye protection) have occurred in the last two decades, training and credentialing is still an area requiring improvement. Europe leads the United States in regard to operator training (EC 2000). According to the American Medical Association, as of 2011, only 27 states had enacted legislation regarding radiation education for fluoroscopy operators. Most guidelines for training in radiation protection and management have come from professional societies (e.g., Society for Interventional Radiology) (Hirshfeld et al. 2005; Stecker et al. 2009; Miller et al. 2010; Chambers et al. 2011), and specific training recommendations are found in NCRP Report No. 168 (NCRP 2008). Several strategies have been developed for both patient and staff safety in FGI and should be considered when setting up a new program or auditing an existing practice (NCRP 1995, 2010; ICRP 2000; Rehani et al. 2010; IAEA 2012; Cousins et al. 2013).
NCRP Reports No. 122 (NCRP 1995), No. 133 (NCRP 2000), and No. 168 (NCRP 2010) provide specific recommendations for radiation monitoring of individuals and current algorithms for estimating effective dose to staff that tend to overestimate effective dose. Equivalent dose to the lens of the eye is assessed typically using a monitor at the collar level outside any radiation protective garments or near the eyes. However, improvements in the understanding of the operational assessment of Hp(3) for diagnostic radiology energies are still necessary. Behrens and Dietze have studied the mechanisms for accurately measuring LDE extensively (Behrens and Dietze 2010, 2011; Behrens 2012) and have provided data that show that DDE dosimeters worn on the trunk or waist far from the eyes underestimate LDE at low photon energies (because they are too thick) and may be worn under a lead apron if in use (thus not evaluating LDE at all). SDE dosimeters worn facing the beam or scatter and worn near the eye (perhaps on the collar or on glasses) should be adequate for photon exposures but may overestimate beta exposures (because they are too thin). If LDE dosimeters are developed, they also must be worn facing the beam or scatter and near the eye, but this would be the only type adequate for both photons and beta exposure.
RADIOACTIVE MATERIAL IN DIAGNOSTIC MEDICINE
Increasing use of radioactive materials in diagnostic imaging, especially positron emission tomography (PET), multimodality imaging (PET/CT, PET/MR), nuclear medicine imaging (e.g., stress tests, scans), and localization studies (e.g., sentinel node, radioactive seed localization), have increased the potential for staff exposures. This is especially true for patient positioning, injection of dosage, and preparation of doses, both in nuclear medicine suites and outside traditional radiology departments. Dose rates from nuclear medicine patients are about 10, 50, or 90 μSv h−1 GBq−1 at 1 m for 99mTc, 131I, or 18F, respectively, and close contact with PET patients can result in 0.5–3 μSv min−1. Whole body doses per patient are ~0.2 μSv for bone scans, 1.5 μSv for MIBI [99mTc-sestamibi (nuclear cardiology) scan] single photon emission computed tomography, and ~5 μSv for PET, with ~1 μSv for injection and up to 4 μSv for escorting or assisting the patient. Preparation and assay of radiopharmaceuticals are associated with the highest occupational exposures in nuclear medicine (up to 5 mSv y−1 whole body and 500 mSv y−1 extremity). Clearly there is a need to develop and implement more advanced shielding, dispensing, assay, and delivery methods. Tungsten shields of ~2 mm for 99mTc and 5 mm for 18F or 90Y can assist in reducing exposures, but they do not guarantee low exposures without effective training, tools, maximized distance over time, and low-dose methods for dispensing.
As shown earlier, extremity monitoring is a necessity in nuclear medicine. The collaborative European Optimization of Radiation Protection for Medical Staff (ORAMED) Project Work Package 4 on extremity dosimetry in nuclear medicine has noted that 35% of workers exceed 250 mSv y−1, and 20% of workers exceed 500 mSv y−1 SDE (Vanhavere 2012). This exceptionally useful ORAMED project has resulted in a detailed assessment of extremity monitoring and found that monitoring the inner base of the index finger of the nondominant hand can correlate with the maximum SDE (typically at the tip of the finger). A rough estimate of the maximum dose is to multiply the inner index finger dose by about six (Vanhavere 2012). Facilities are encouraged to assess both monitoring and protection practices as nuclear medicine imaging and therapy procedures continue to be developed and increase in use.
NOVEL USES IN MEDICINE
There is increasing use of novel treatment approaches using beta emitters, targeted alpha particle therapy, theragnostic radiolabeled compounds (with both a diagnostic and therapeutic purpose), and intraoperative radioactive material use for brachytherapy or PET-guided interventions and surgery that represent new challenges for medical staff. In addition, several institutions are building in-house cyclotron and radiopharmaceutical facilities and developing nontraditional PET isotopes such as 64Cu, 68Ga, 86Y, 89Zr, and 124I that involve emission of high-energy gamma rays, in addition to 0.511 MeV annihilation photons. These uses present challenges for occupational exposures with respect to shielding and radiation safety issues (Holland et al. 2010).
There is a blurring of responsibility boundaries in the novel uses of radiation in medicine, with overlapping among interventional radiology, nuclear medicine, radiology, radiation oncology, and medical physics. Novel uses of radiation in medicine require a reassessment of radiation safety practices and responsibilities for staff protection, dose monitoring, and control.
This review of exposed medical staff has shown that doses have increased over the past decade as there is an increased need for and use of radiation in medicine. While current average doses are well within current limits, some individuals could exceed 20 mSv y−1 DDE or LDE (if unprotected), and there is the potential for some workers to exceed 500 mSv y−1 SDE without careful assessment and protection. Nuclear medicine radiochemistry and patient dose preparation present challenging opportunities for improved dose control. In addition FGI procedures continue to represent an important area for careful protection implementation. Optimization of radiation protection in the medical setting should include tried and true principles of justification, optimization, and limitation with emphasis on training, credentialing, planning, and quality management. As newer and developing uses of radiation in medicine are tested and implemented, it is important to consider effective dosimetric monitoring, lens of eye doses, extremity doses, novel uses, and novel radionuclide characteristics. An ongoing assessment of current and future patterns of use for radiation in medicine is an essential activity to assist in prioritizing limited resources toward staff protection.
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