1. Introduction
According to census data of the Chinese Society of Nuclear Medicine in 2009, 2011 and 2013 [
1,
2,
3], radionuclide therapy has been developing very rapidly, and staff engaged in the work of nuclear medicine has also increased in recent years. By the end of 2013 [
3], there were 610 medical institutions engaged in radionuclide therapy and a total of 8678 workers were working in the field of diagnostic and therapeutic nuclear medicine in China.
Practices in nuclear medicine involve the manipulation of many radiopharmaceuticals labeled with different unsealed radionuclides, both for diagnostic and therapeutic purposes, and may pose significant risks of internal contaminations to the workers. From a radiation protection point of view, the accurate assessment of the dose of internal exposure that nuclear medicine staff receives is as important as the accurate assessment of the dose of external exposure. Although it is considered that doses from radionuclide intakes in nuclear medicine centers are usually lower than doses from external exposure, the risks related with intakes should be estimated in every case and, if it is necessary, workers involved in the handling of unsealed sources should be monitored routinely, ensuring that the doses that workers receive are maintained as low as reasonably achievable. Internal monitoring should also be performed in case of accident or when suspected inhalation or ingestion intake occurs.
According to the 2014 Basic Safety Standards [
4], routine internal monitoring should be conducted on workers in controlled areas where there are risks associated with incorporation of radionuclides. ICRP in its publication 78 [
5] recommends that thyroid monitoring should be carried out in interval of 7, 14 or 30 days for those workers engaged in iodine therapy, depending on the workload, sensitivity of the detection system and the methodology available for the implementation of a monitoring program. Relevant standards in China [
6,
7] only stipulate a general and vague principle that workers engaged in producing a large number of radioactive isotopes and processing a large number of
131I labeled radiopharmaceuticals, and workers possibly internally contaminated by radioactive material, should be subjected to conventional internal individual monitoring. The internal monitoring of nuclear medicine staff, however, has not been carried out in most institutions in China thus far. The need for individual internal monitoring is dependent on the quantity and type of radioactive material present, the physical and chemical form of the radioactive material, the type of containment used, the practices implemented, as well as the general working conditions.
It is rather difficult to determine whether it is necessary to conduct internal monitoring for intakes of radioactive material for a worker. In 1999, the International Atomic Energy Agency (IAEA) published a safety guide (No.RS-G-1.2) [
8] which aims to provide a set of criteria to be taken into account in order to determine whether or not an internal monitoring program is needed. The criteria are based on the evaluation of several factors to estimate the dose due to intake of radionuclides in the workplace. The decision on implementing internal monitoring is carried out when the evaluation results in an annual committed effective dose equal to or higher than 1 mSv.
In recent years, more and more attention has been focused on the internal exposure of nuclear medicine staff. Research in this area is divided into two parts: the first part is the study of the committed effective dose resulting from intake of radionuclides. A series of studies have been carried out on the internal exposure of nuclear medical staff by air sampling [
9,
10,
11,
12,
13], direct measurement [
14,
15,
16,
17,
18] or biological materials sample analysis [
17,
19,
20]. The second part is the study of the necessity of internal monitoring for nuclear medicine staff. Researchers in Chile [
21], Portugal [
22], Brazil [
23] have carried out some research based on the aforementioned methodology to determine whether staff should be subjected to internal monitoring. Since the 1980s, there have been only a few studies and a small number of reports on the work of internal exposure in the field of nuclear medicine in China [
9,
18]. This work reports the application of criteria proposed by the IAEA to determine for the first time whether internal monitoring is needed for nuclear medicine staff in a Chinese hospital.
4. Discussion
All staff members of nuclear medicine in a specialized provincial cancer hospital were interviewed, radionuclides handling data were obtained to determine the necessity of internal monitoring. Due to the large workload in this hospital, it was found to be necessary to monitor all 18 staff members.
There are many similar studies in other countries. Dantas
et al. [
23] observed that
131I presents a high risk, and internal monitoring of
131I should be included in the radiological protection plan, especially when used in therapeutic applications where the quantity of activity administered to each patient is usually several tens of millicuries. Bento
et al. [
22] found that 71.9% of all staff from nuclear medicine centers from all the participating institutions should be integrated in a routine monitoring program for internal contaminations. Astudillo
et al. [
21] also conducted a similar research in three nuclear medicine centers on the basis of IAEA criteria, and found it is necessary to carry out a routine monitoring program for five workers who handle
131I and three workers who handle
99mTc, where there were in total seven workers who handled
131I and five workers who handled
99mTc. Hence more than half of the workers investigated should be subjected to internal monitoring.
This study shows that it is necessary to carry out routine internal monitoring for all of the 18 staff members. The ratio might be higher compared with other studies, highlighting the necessity of internal monitoring and importance of internal contamination. To the best of our knowledge, this work is the first of its kind to determine whether internal monitoring is needed for nuclear medicine staff in a Chinese hospital by applying the criteria proposed by the IAEA in 1999, and so fills a gap in China.
It should be noted, however, that the decision factor values related to a certain operation, and units of mSv do not represent the dose truly received by the worker performing such an operation but represent a potential committed effective dose that can occur whilst performing that operation. As can be seen from Equation (2), the greater activity handled by the worker (Aj), the greater risk of the operation (fhs) and when less protective measures are employed (fps), the greater incorporation likelihood. Doctors and technicians performing 131I automatic dose fractionation got the dj of 47. In addition, they were responsible for 99mTc elution, labelling and dose fractionation, and the corresponding dj of 99mTc was 24 (the sum of 20, 2.0 and 2.0). The nurses injecting 99mTc got the lowest dj of 4.0. Compared with nurses injecting 99mTc only, doctors and technicians reached the higher “decision factor” (dj), and so probably received a greater committed effective dose due to a larger workload than nurses. Considering the hospital is a large and very busy provincial cancer center, the results are understandable. To balance the staffing and the workload to minimize the dose received by the workers are challenges to be addressed.
China is a country with a vast territory possessing numerous hospitals, and the level and scale of the hospitals, the workload of the workers and the protective measures employed in the field of nuclear medicine vary around the country. We can expect very substantial differences for dj, as well as the committed effective dose among hospitals and workers around the country. This study only investigated 18 nuclear medicine staff of one specialized hospital, and further investigation on the necessity of the internal monitoring for nuclear medicine staff in different areas and different levels of hospitals in China is urgently needed.
Additionally, considering that the decision factor assessment may overestimate the committed effective dose [
21], internal monitoring work is needed to solve the problem of what dose from internal radiation the staff actually receive. Internal monitoring can be conducted in three ways: direct measurement of radionuclide in whole body or some organs, excreta or other biological samples analysis and air sampling analysis, respectively [
6]. Compared with the external individual monitoring, the method of internal individual monitoring is diversified, and the result is easily influenced by many factors. The published literature reports [
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20] regarding internal exposure in the field of nuclear medicine mainly did research on commonly used nuclides
131I,
99mTc and
18F, and among them, internal exposure induced by
131I were most studied and reported.
It is important to note that since the 1980s, the coverage of external individual monitoring of radiation workers in China has been increasing steadily [
26], and China has also successively formulated and issued a series of standards [
6,
27,
28] on internal individual monitoring, but for various reasons, internal individual monitoring has not been currently implemented in most areas of China. The 18 workers in the study wear under-apron thermoluminescent LiF (Mg, Cu, P) dosimeters on the chest when working in order to obtain the external dose, and the criterion used for external dose monitoring is GBZ 128-2002 [
29]. Their recorded external doses were less than 1 mSv in a year. An investigation [
9] showed that internal exposure caused by the inhalation of radioactive aerosol is an important part of occupational exposure of the nuclear medical staff, so we should attach great importance to it. Relevant research work on internal monitoring and dose assessment are needed [
26]. Methods of internal monitoring that are suitable to Chinese situation are under in-depth exploration.
Some limitations in the study must be acknowledged. First, a work model of taking turns was employed in this nuclear medicine department to try to achieve a balanced workload among these workers. Daily operation records of each staff of all the year round are not well-kept, and we can only find records of the last two months. Hence it is difficult to get accurate activities of each operation and each worker. We have adopted the average values. Theoretically, the hospital should have accurate official records of radionuclides it purchased. Second, most of the patients with thyroid cancer were administered 3.7 × 109 Bq131I, and only a few of them were administered a dose of 5.6 × 109 Bq or 7.4 × 109 Bq, we simplified the calculation and adopted a dose of 3.7 × 109 Bq for all the patients when calculating, which caused certain underestimation. Third, after doctors or technicians performed 99mTc dose fractionation, the 99mTc labeled radiopharmaceutical were injected into the bodies of patients in minutes. Due to the radioactive decay of 99mTc (T1/2 = 6.02 h), the activity of injection is slightly smaller than the activity of dose fractionation. Since minutes are much shorter than 6.02 h, we ignore the decay of 99mTc in minutes and approximately consider that the activity of injection and dose fractionation are the same. In summary, it is rather difficult to obtain relatively accurate handling activity for each nuclide of each worker. Daily operation records of each staff member for all the year round are not well-preserved, and it is difficult to accurately estimate activities of each operation and each worker. Also, the criteria suggested by IAEA relate solely to inhalation of radionuclides, so other intake routes including ingestion of radionuclides are not taken into account in this study. Eating and drinking are strictly prohibited in China when handling radionuclides, but it may be necessary to confirm this.