Introduction
In hospitals, patients injected with radiopharmaceuticals become mobile sources of radiation, often referred to as ‘walking radioisotopes.’ Even non-radiation workers who conduct radiological diagnostics in radiation-controlled areas may be exposed to radiation when attending to these patients. In Japan, laws and regulations related to radiation safety management in the medical field include the Law Concerning Act on the Regulation of Radioisotopes, Regulation on Prevention of Ionizing Radiation Hazards, and Enforcement Regulations on the Medical Care Act [1]. These legislations recommend that radiation dose limits for radiation workers be aligned with those set forth by the International Commission on Radiological Protection (ICRP) [2–5]. However, the dose limits for the general public, patient attendants, and medical staff slightly differ from those recommended by the ICRP and International Atomic Energy Agency [6–8].
The Cancer Institute Hospital, operated by the Japanese Foundation for Cancer Research (JFCR), has 700 inpatient beds, provides an average of approximately 1,400 outpatient visits, and employs around 1,200 staff members. It specializes in cancer care and conducts numerous nuclear medicine examinations for cancer detection [9]. Within the nuclear medicine department, approximately 950 examinations are performed each month, predominantly consisting of bone scintigraphy and positron emission tomography (PET). This study assessed the radiation exposure of staff members (excluding radiation workers) from patients injected with radioactive isotopes (RI) and evaluated safety measures [10, 11].
In Japan, in addition to physicians, several types of professionals, including clinical laboratory technicians, can perform ultrasound examinations. In 2007, there were approximately 270,000 registered clinical laboratory technicians and public health laboratory technicians; only 60,000 were actively employed in hospitals and clinics. Clinical laboratory technicians are considered as radiation workers when involved in angiography or performing specimen tests using RI, leading to persistent concerns regarding radiation protection [12, 13]. This study assessed the radiation exposure levels of clinical laboratory technicians who were not designated as radiation workers but were involved in ultrasonography to determine the radiation exposure levels of non-radiation workers.
Materials and Methods
The type, number, timing, and sequence of RI-injected examinations performed between January and June 2008 in the nuclear medicine department of the Cancer Institute Hospital of JFCR were recorded. Then, the type and time of ultrasound examinations performed during the same period in the ultrasound laboratory were obtained. Subsequently, instances of same-day examinations performed on the same patient were identified, focusing on the temporal relationship between these procedures.
Between January and June 2008, 14 clinical laboratory technicians employed in ultrasound laboratories, representing non-radiation workers in hospitals (treated as the general population in Japan), were equipped with personal radiation dosimeters to measure their radiation exposure levels. These dosimeters, in the form of single optically stimulated luminescence dosimeters, were worn on the chest by men and on the abdomen by women, assuming uniform exposure. Furthermore, estimated effective dose of radiological technologists working in the department of nuclear medicine were recorded for comparison during the same period.
The estimated effective dose for clinical laboratory technicians, representing non-radiation workers and serving as representatives of the general workforce, were assessed to ensure legal compliance in Japan. Furthermore, a set of precautions was recommended to be observed when conducting nuclear medicine examinations and ultrasound examinations on the same day, as well as precautions for hospital staff when interacting with RI-administered patients.
Results and Discussion
To determine the number of examinations conducted in the nuclear medicine department of the Cancer Institute Hospital of JFCR, the number of scheduled examinations over the course of 1 week in 2008 was recorded (Table 1). Notably, F-18 fluorodeoxyglucose-PET and bone scintigraphy emerged as the most common examinations. Subsequently, data were analyzed regarding the types, quantities, timings, and sequences of tests performed between January and June 2008 that involved radiopharmaceutical injection (Table 2). The annual number of tests was determined via multiplication of the observed figure by two. Cumulatively, all types of tests were administered in over 11,000 instances, averaging approximately 950 nuclear medicine tests per month. In particular, around 5,400 bone scintigraphy and 4,500 PET tests were performed annually.
The types of examinations conducted in ultrasound laboratories in 2007 were recorded (Table 3). This dataset comprised approximately 53,000 examinations conducted throughout the year. Considering that the study hospital specialized in cancer care, a substantial proportion of these examinations focused on breast cancer. Furthermore, data concerning the types and durations of ultrasound examinations performed between January and June 2008 were recorded. As this survey only covers the first half of 2008, the results from the previous year, 2007, were used as a method for obtaining the number of cases for the whole year. This confirmed that doubling the data for the first half of 2008 is approximately the same as the number for 2007. In this survey, there were no exclusion criteria and all cases were included, and body size and dosage were not considered.
Based on the aforementioned results, the temporal relationship between examinations carried out on the same patient on the same day was established (Fig. 1). During the study period, 22,176 ultrasound examinations and 5,798 nuclear medicine examinations were conducted. Of these, 3,055 individuals underwent both types of tests, comprising 5,223 ultrasound examinations and 3,778 nuclear medicine examinations. Moreover, 1,614 individuals underwent both tests on the same day, indicating that 7.3% of those who underwent ultrasound examinations and 27.8% of those who underwent nuclear medicine examinations had multiple examinations on the same day.
There were 19 cases in which patients underwent two bone scintigraphy tests, four cases in which patients underwent three bone scintigraphy tests, and no instances in which patients underwent four or more bone scintigraphy tests during the 6-month investigation period. Similarly, 57 patients underwent two PET examinations, two patients underwent three PET examinations, and no patient underwent more than four or more PET examinations.
The 1,614 instances of dual examinations on the same day were categorized by type, considering the implications of radiation exposure (Fig. 2). The start time was set to 0 when the RI was administered. In totals, 1,366 cases involved both bone scintigraphy and ultrasound examinations conducted on the same day. Among these, 145 ultrasound examinations were conducted prior to RI administration, posing no radiation exposure risk to the clinical laboratory technologist. However, 772 ultrasound examinations were conducted after RI injection, during the 2–3 hours waiting period for drug accumulation. Radiation exposure during an ultrasound examination performed within this waiting period may be higher than appropriate/allowed for the clinical laboratory technologist of a non-radiation worker. Furthermore, 449 patients underwent ultrasound examination after RI image acquisition. Considering that the half-life of Tc-99m is 6 hours, radioactivity levels may remain relatively high during this period, resulting in moderate radiation exposure for the clinical laboratory technologist. Notably, ultrasonography of the lower abdomen requires a full bladder, whereas RI examinations typically involve RI excretion through urine, leading to significant RI accumulation in the urinary bladder and consequently, heightened radiation exposure for medical staff.
A diagram was generated to illustrate the relationship between the time elapsed from RI administration ‘intravenous (i.v.)’ and the nuclear medicine acquisition time for the 1,614 cases (Fig. 3). In this diagram, ‘Pre’ denotes the group in which ultrasound was performed before RI administration, ‘Wait’ represents the group in which ultrasound was performed during the waiting period for drug accumulation after RI administration, and ‘Post’ refers to the group in which ultrasound was performed after RI image acquisition. Furthermore, the figure depicts the relationship between the time of nuclear medicine acquisition and the time of ultrasound examinations according to the time of RI administration (Fig. 4). In this figure, the red dotted line indicates the high-risk group for radiation exposure among clinical laboratory technicians.
Similar to bone scintigraphy, PET examinations were categorized according to the temporal relationship with ultrasound examinations. In total, 226 PET and ultrasound examinations were conducted on the same day. Among these, 174 ultrasound examinations were conducted prior to RI administration, posing no radiation exposure risk to clinical laboratory technicians. Due to the examination protocol, patients were required to rest during the waiting period and were not permitted to move to the ultrasound examination room. Thus, there is no radiation exposure for clinical laboratory technicians during this time. Furthermore, 52 cases underwent ultrasound examination after PET image acquisition. Considering that there is a waiting period for attenuation even after PET, the ultrasound examination was conducted after an adequate duration of time had passed. Although F-18, used in PET, has a very high-energy level, its half-life is only 110 minutes. Moreover, considering the small number of ultrasound examinations performed, the effects of PET examinations were likely minimal.
The dose measurements for the 14 clinical laboratory technicians wearing personal dosimeters indicated effective doses ranging from 0 to 0.3 mSv/mo. Some staff recorded monthly doses ranging from 0.1 to 0.2 mSv/mo. When the results were annualized, eight of 14 (57%) technicians exceeded the public effective dose limit of 1 mSv/yr. The maximum recorded dose was 1.8 mSv/yr (Table 4). In 2008, work was not segregated by gender; however, as of 2024, ultrasound examinations of the female breast are often conducted by female clinical laboratory technicians in Japan. Notably, 80% of the female clinical laboratory technicians recorded estimated effective doses exceeding 1 mSv. These findings are attributable to potential issues regarding body position and posture during ultrasound examinations, as well as dosimeter placement. The clinical laboratory technicians often performed ultrasound examinations in close proximity to the bed on which the RI-administered patient was lying. Moreover, there is a high concentration of RI in the bladder when the patient holds their urine during the ultrasound examination. Furthermore, male clinical laboratory technicians typically wear a personal radiation dosimeter on their chest, whereas female technicians wear it on their abdomen. This may explain why estimated effective doses recorded by female technicians were higher than those recorded by male technicians.
For comparison, radiation dose data for radiological technologists working in nuclear medicine departments were analyzed (Table 5). Radiological technologists are considered as radiation workers, and their occupational radiation exposure is regulated by law. The average estimated effective dose for radiological technologists during the same period was 2.6 mSv, with a maximum calculated annual effective dose of 4.8 mSv. For reference, the effective dose for a radiological technologist responsible for radiation therapy and a nurse caring for a patient undergoing I-125 treatment is negligibly low and below the detection limit of the personal dosimeter (indicating 0 mSv/mo).
In the Cancer Institute Hospital of JFCR, ultrasound examinations were commonly performed after RI administration for bone scintigraphy and PET acquisitions, which likely reflects the shared goal of patients and physicians to optimize hospital visits by scheduling additional examinations for patients receiving RI while they wait for nuclear medicine examinations. The inclusion of clinical laboratory technicians, who spend a lot of time in contact with patients during ultrasound examinations, in this study likely had a considerable impact on the results. Although a clinical laboratory technician only has about 15 to 30 minutes to deal with each patient, the time for radiation exposure naturally increases when examining many patients during a day’s work time. It is estimated that effective dose for clinical laboratory technicians exceeds the public effective dose limit, potentially reaching 2 mSv/yr in certain cases, which surpasses the dose limit for pregnancy. Although the findings of this study may not be generalizable to all hospital employees, it is crucial to recognize that employees who spend extended periods in close contact with patients may be exposed to radiation, even if they are not considered as radiation workers. Considering these findings, it is imperative to implement radiation dosimetry and establish protective measures for medical staff.
Conclusion
Radiation exposure control for medical staff and patients within the radiation department is a well-established practice in all healthcare facilities. However, there is increasing recognition of the importance of implementing similar controls for radiation use outside of dedicated radiation departments [14–18]. Furthermore, some patients administered radiopharmaceuticals may effectively become mobile sources of radiation. Therefore, this study estimated effective doses for medical staff who are not considered as radiation workers by measuring doses from these RI-administered patients, particularly focusing on clinical laboratory technicians working in a specialized cancer hospital.
This study analyzed the types, number, timing, and sequence of examinations requiring RI administration performed in the nuclear medicine department of Cancer Institute Hospital of JFCR between January and June 2008. Furthermore, the type and timing of ultrasound examinations during the same period were recorded. These data were used to identify temporal relationships between examinations performed on the same day in the same patient.
Annually, more than 11,000 examinations are conducted, comprising approximately 950 nuclear medicine tests per month. Approximately 5,400 bone scintigraphy examinations, 4,500 PET examinations, and almost 53,000 ultrasound examinations are conducted annually. During the study period, 3,055 patients underwent both examinations, resulting in 5,223 ultrasound examinations and 3,778 nuclear medicine examinations. Notably, 1,614 cases involved both tests on the same day, indicating that 7.3% of those who underwent ultrasound examinations and 27.8% of those who underwent nuclear medicine examinations had multiple examinations on the same day. Furthermore, 772 ultrasound examinations were conducted within 2–3 hours after RI administration for bone scintigraphy examinations while waiting for drug accumulation. During this waiting period, the radiation exposure of clinical laboratory technicians is likely to be increased. Furthermore, 449 patients underwent ultrasound examination after RI image acquisition. Considering that the half-life of Tc-99m is 6 hours, radioactivity remains relatively high, resulting in moderate radiation exposure for the technicians. Although previous reports have indicated low radiation exposure for clinical technicians during echocardiography after cardiac scintigraphy using Tl-201, the present study was conducted in a cancer hospital, where cardiac scintigraphy and echocardiography tests are rarely performed [19].
The dose measurements for the 14 clinical laboratory technicians wearing personal dosimeters indicated effective doses ranging from 0 to 0.3 mSv/mo. Some staff recorded monthly doses of 0.1–0.2 mSv/mo in any given month. When the results were annualized, eight of 14 technicians (57%) exceeded the public effective dose limit of 1 mSv/yr. The maximum calculated dose was 1.8 mSv/yr.
The present study was conducted at a specialized cancer hospital, where numerous PET examinations emitting high-energy radiation were performed, posing significant radiation exposure risk to medical staff (radiological technologists in charge of nuclear medicine). Although this risk could not be conclusively demonstrated through the exposure doses of clinical laboratory technicians responsible for ultrasound examinations, the presence of patients carrying sufficient levels of RI within the hospital remains a concern. Furthermore, there is a prevalent practice of efficiently utilizing the waiting time for nuclear medicine examinations, particularly bone scintigraphy tests, which require a lengthy waiting period of 3 hours after RI administration. This practice arises from the shared desire of patients and physicians to maximize the efficiency of hospital visits by scheduling additional examinations during this waiting period or by performing multiple examinations on the same day. Although new knowledge has been discovered and optimized over time, there has been no change in the regulatory requirements for effective dose control between 2008 and 2025. Therefore, regardless of whether an individual is considered a radiation worker or non-worker, radiation protection remains crucial. Nevertheless, radiation protection of medical staff as occupational exposure needs to be a priority, as patients change daily with benefits. In terms of staff protection, if examinations are to be conducted on the same day, there should be adequate time intervals between those examinations; careful consideration should be given to the sequence in which they are performed.
The limitations of the study were discussed; data from 2008 were used because the survey was conducted that year as part of the work environment survey. The same conditions may not necessarily apply at present. It is not known if similar radiation exposure situations exist among clinical laboratory technicians.
According to a report by the Japan Radioisotope Association, 1,417,700 single-photon examinations and 414,300 PET examinations were performed in Japan as a whole in 2007. In 2022, 1,113,500 single-photon examinations and 700,900 PET examinations were performed [20]. Although the total number was almost the same, single-photon examinations decreased by about 20% and PET examinations increased by about 70%, indicating a shift in the content of examinations over time. Since the purpose of this paper is to assess the effective dose of medical staff during single-photon examinations, it it estimated that if an effective dose survey were conducted at present, the effective dose would be approximately 20% lower, reflecting the decrease in the number of examinations. However, it is not possible to determine whether this can be estimated simply from the number of examinations without current measured data.
In addition, the details of the exposure status of individual clinical laboratory technicians are unknown because detailed surveys of the number of examinations, types of procedures, and time spent in close proximity to patients injected with radiopharmaceuticals have not been conducted. A shortcoming of this study is that the available data do not include detailed data on individual persons, and thus the details of their exposure status cannot be ascertained. In order to conduct a more in-depth and detailed investigation in the present day, it is necessary to obtain detailed exposure data by ascertaining the detailed behavior of each individual, which requires an ethics review application.
However, there are no reports from Japan or elsewhere in the world that measures doses for individuals other than radiation workers in actual situations. Even in 2024, there have been no reports of radiation doses for clinical laboratory technicians who are not certified as radiation workers. The 2008 data presented here are the first in the world. Although this study used data from 2008, it may be helpful for those investigating the current situation (e.g., the Society of Clinical Laboratory Medicine). In other words, this study presents the only available data in the world on non-radiation health care workers and serves as an important reference.
It is expected that many researchers will conduct a detailed studies on the current situation of various health care workers, and such data will contribute to radiation safety.
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