Current situations and discussions in Japan in relation to the new occupational equivalent dose limit for the lens of the eye

In 1994, the US Food and Drug Administration published Avoidance of Serious x-ray-Induced Skin Injuries to Patients during Fluoroscopically-Guided Procedures, in order to deal with skin injuries to patients owing to x-rays used in the medical field, in consideration of an increasing number of fluoroscopically-guided IVR procedures [19]. In Japan, in 1995, the Radiation Protection Committee of the Japanese Radiological Society took this issue into account in a statement regarding exposure of patients and physicians during IVR procedures [20].

In 2000, ICRP issued Publication 85 to reduce and protect against exposure of the skin and eye in patients and medical staff [21]. Moreover, in Japan, the relationship between dose and its potential impact on the skin and eye during fluoroscopically-guided procedures and cases of skin injuries in Japanese patients was described in 2004 [22], highlighting the importance of estimating the exposure of the skin of patients. In 2006, a Joint Working Group of the Japan Circulation Society (JCS) published the Guideline for Radiation Safety in Interventional Cardiology (IC) (revised in 2013) [23, 24]. These guidelines addressed general issues (e.g., basic information about radiation exposure and its health effects) and specific issues (e.g., dose response for skin injuries and strategies for reducing exposure in medical personnel and during the treatment of pregnant patients). Other specific issues include the necessity of protecting the eyes and the thyroid of medical staff by using safeguards, and the importance of standing position, time, and the choice of protective clothing.

In 2000, exposure of patients and operators was measured during the course of 39 transcatheter arterial embolization procedures at five medical facilities in Japan [25]. The average equivalent doses to the skin of the patients and to the skin between the eyebrows of the operators were 973 ± 681 mSv/procedure and 0.04 ± 0.04 mSv/procedure, respectively.

In 2003, TLDs and RPLDs were used to measure the skin surface doses for a physician during endovascular treatment for brain disease [26]. The mean surface skin doses at the left and right corners of the eye and the neck of the physician were 0.012 mGy/procedure, 0.102 mGy/procedure, and 0.015 mGy/procedure, respectively, under the device mode with an average tube voltage of 84.5 kV, an average tube current of 12.9 mA and a total exposure time of 15.5 min. The doses to the fingers and eye (between the eyebrows) were measured at four facilities using TLDs [27]. For procedures such as catheter placement, embolization treatment of a cerebral aneurysm (ETCA), and percutaneous transluminal coronary angioplasty (PTCA), the dose to the lens (H_p(0.07)) during PTCA procedures was the highest. The average dose and maximum dose during PTCA procedures were 0.3 mSv/procedure and 2.1 mSv/procedure, respectively.

A dose measurement system was developed where the skin surface dose distribution in a patient body is evaluated with more than 50 RPLDs [28, 29]. In that study, direct measurement of the patient dose as well as dose mapping could be performed easily where patients and operators had a vest and head cap to which RPLDs were attached. The average entrance skin doses at the left and right eye positions of the operator, taken over 25 therapeutic IVR procedures, were 0.254 ± 0.338 mGy and 0.028 ± 0.032 mGy, respectively, for a mean fluoroscopic time of 44.1 ± 31.2 min [28]. Recently, a study using the same method was performed on the diagnostic reference level and exposure for medical staff during IVR procedures for treating brain disease [30].

In 2014, a multicenter study was performed by the Japanese Society of Radiological Technology (JSRT) to evaluate the lens dose in medical staff, such as physicians and nurses, during non-vascular endoscopic IVR procedures at Japanese medical facilities [31, 32]. In that study, the medical staff wore protective glasses with small OSLDs, and the lens dose during procedures was measured for one month at 11 medical facilities [32]. At the 2015 autumn meeting of the JSRT, problems with the radiation management for the lens, correction coefficients estimated from measurements at the neck to the equivalent dose to the lens, and a survey on lens dose of operators during cerebral aneurysm embolization were reported [33–35].

There have been several studies on occupational exposure that have also conducted dose measurement during IVR procedures [36–38], where photon spectra were considered for more precise evaluation of the lens dose and for the standardization spectra for the test of dosemeters.

A performance of radiation protective eyewear used in the medical field was evaluated, where the dose reduction rates for 0.07 mmPb and 0.75 mmPb protective eyewear were measured using an over-tube type of fluoroscopy device [39]. Depending on the type of eyewear, the reduction rate of the left eye surface dose of a phantom ranged between 9% and 62% when the tube was located on the left side of the phantoms. These results indicate that the dose reduction rate of the lens dose by eyewear and the distance between the eyewear and the eye are important factors in the reduction of lens dose.

Other studies have been performed on the exposure of Japanese medical staff during computed tomography (CT) angiography procedures [36, 37]. The dose distribution of scattered radiation near the gantry was measured using OSLDs during multi-detector row CT (MDCT) procedures within the CT rooms. The maximum dose, H_p(10), at 50 cm from the gantry and 150 cm above the floor, was 1.47 mSv/examination [40]. The skin surface dose was measured using RPLDs during CT-guided biopsy procedures for lung cancer [41]. These annual equivalent doses to the skin of the fingers, with and without a protector (e.g., a needle guide) were less than 0.0047 mSv and 0.63 mSv, respectively. The annual equivalent doses to the lens measured at a position between the eyebrows, with and without the protector, were 0.025 mSv and 0.02 mSv, respectively. The equivalent dose to the lens was extremely low under CT-guided biopsy procedures [41]. Table 1 lists doses for medical staff and medical procedures.

Knowledge of exposure of the lens during nuclear medicine diagnostics has not yet been accumulated sufficiently to discuss the equivalent dose to the lens in Japan. There are calculated estimations for ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) used for a positron emission tomography (PET) at medical facilities [42, 43]. In those studies, it was estimated that the equivalent dose to the skin (H_p(0.07)), at 60 cm from an injection tube became 20% of the dose at 20 cm from the tube [42], and the calculated equivalent dose to the lens (H_p(3)) was 70 μSv/min at 5 cm from the tube [43]. In 2008, the Medical Science and Pharmaceutical Committee of the Japan Radioisotope Association (JRIA) prepared a questionnaire to determine the occupational dose at nuclear medical facilities [44]. The effective doses for doctors, radiological technologists, pharmacists, cyclotron operators, nuclear medicine technologists (who produce or synthesize nuclear medicine) and nurses were estimated from doses measured using RPLDs and electronic dosemeters at the facilities. Of these, radiological technologists received the highest exposure, the average effective dose being 110 μSv/month. The average effective dose for the cyclotron operator and nurse was 100 μSv/month. The dose for the full-time medical doctor and pharmacist was 25–40 μSv/month. At most nuclear medical facilities, γ emitters (e.g., ⁶⁷Ga, ⁹⁰Sr, ^99mTc, ¹¹¹In, ¹³¹I, and ²⁰⁴Tl) or positron emitters (e.g., ¹⁸F) are used. Thus, the lens dose would be comparable to the effective dose. The annual equivalent dose to the lens in radiological technologists was found to slightly exceed 1 mSv.

The JRIA working group for brachytherapy promotion published a guideline for safety management of permanent brachytherapy using ¹²⁵I [45]. This guideline aims to comply with national regulations related to brachytherapy patient releases from medical facilities [46, 47], and follows ICRP Publication 98 [48]. For the brachytherapy, about 80 pieces of ¹²⁵I seeds (13.1 MBq/seed) are used for a patient. In this case, the effective doses for medical doctors and nurses, estimated based on distance from a device, workload and activity of radiation sources, were 0.0033 mSv/procedure and 0.0015 mSv/procedure, respectively. The equivalent doses to the skin of medical doctors and nurses were 0.041 mSv/procedure and 0.007 mSv/procedure, respectively [45]. The effective dose and equivalent dose to the skin during preloading work were 0.0229 mSv/procedure and 0.206 mSv/procedure, respectively [45]. This guideline did not deal with the equivalent dose to the lens or radiation protection of the lens, but only recommended that medical staff should treat radiation sources with tweezers to protect the skin, reduce operating times, and shield radiations.

Japan has 42 operable nuclear power reactors (22 boiling water reactors, or BWRs, and 20 pressurized water reactors, or PWRs). These reactors are in 16 locations and are operated by 10 electric power companies [49, 50]. After the 1F-NPP accident in March 2011, the NRA in Japan issued new safety standards [49, 50]. The licensees had to take appropriate measures to restart the plants. Thus, no nuclear reactors were operated from 15 September 2013 to 11 August 2015. Currently, there are 13 units in the restart process, and 12 units are licensed from the NRA. Five reactors were in operation as of June 2017 (Sendai-1, Sendai-2, Ikata-3, Takahama 3 and Takahama 4 [49, 50]. In addition, 15 reactors, including 6 BWR reactors at 1F-NPP, are under decommissioning or are currently under review for decommissioning plans.

The occupational external exposure for workers at nuclear facilities is due to γ-rays emitted from activation products from corrosion such as ⁶⁰Co, ⁵⁸Co and ⁵⁴Mn, and air-borne activation products such as ¹⁶N formed in the reactor core and preliminary cooling system during operation periods of NPPs, nuclear research and development facilities [51]. In BWRs, high-dose radiation fields are formed around steam turbine generators. However, the half-lives of these radionuclides are short. Thus, the external exposure of workers during regular inspections is mainly due to 0.6–1.3 MeV γ radiation emitted from activation products such as ⁶⁰Co, ⁵⁸Co and ⁵⁴Mn that remain in the cooling systems and inside the plumbing of the system after the reactors are stopped [51]. Data for workers constantly exposed to radiation at nuclear facilities, such as radiation exposure doses, are registered in the Radiation Dose Registration Center of Radiation Effects Association.

In the NRA Act, the licensees are mandated to control the dose for radiation workers at nuclear facilities and to report the dose management to the NRA twice a year [52]. In FY 2014, beginning in April 2014, a total of 68 550 radiation workers were employed at commercial NPPs, among whom the number of employees at electric power companies and that of others such as contractors were 9891 and 58 659, respectively. The percentage of female workers was about 1% of total radiation workers [53]. The effective dose is assessed from H_p(10) measured at chest level. In dose management reports [52–54], only the effective dose distribution for the workers has been indicated. In NPPs, the workers are usually exposed to γ radiation. The exposure of the workers is approximately homogenous under the planned exposure situation, although exposure from the anterior part of the body is slightly predominant [55, 56]. Thus, this value would be considered comparable to the equivalent dose to the lens.

Figure 5 shows a trend in collective effective dose and the number of radiation workers employed at commercial NPPs for a decade, over FYs 2005–2014 [53]. Figure 6 shows the trend in average effective dose for the workers. Figure 7 shows the dose distributions of electric company employees and others in FY 2009. The dose received during emergency and recovery work during the 1F-NPP accident is included between FY 2010 and FY 2014. The collective dose ranged between 67 and 84 person-Sv in FY 2005–2009. The average effective doses for workers in BWRs and PWRs were 0.8–1.0 mSv and 1.1–1.3 mSv, respectively, in FY 2005–2009, during which time the maximum effective dose did not exceed 20 mSv/year. The effective dose for female workers was also under 5 mSv/3 months [53]. The average dose for the workers in BWRs in FY 2010–2014 was about 2 to 3 times as high as the dose before FY 2010. In FY 2012, the maximum effective dose was 54.1 mSv/year, although the emergency dose limit is 100 mSv (it was raised from 100 mSv to 250 mSv on 14 March 2011, and reduced from 250 mSv to 100 mSv on 1 November 2011). For female workers, the effective dose is also under 5 mSv/3 months except for those at 1F-NPP.

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In the statistical dose data reported by the Radiation Dose Registration Center [53], the collective dose, average effective dose, and maximum dose for the workers were 87.0152 person-Sv (71 142 persons), 1.2 mSv/year, and 43.2 mSv/year in FY 2015, respectively.

The workers are exposed to β or inhomogeneous γ radiation during maintenance work. In these radiation fields, the workers usually wear protective face masks to prevent internal exposure, and active dosemeters, such as electronic dosemeters [57]. Thus, it must be noted that these values are not considered comparable to the equivalent dose to the lens in these radiation fields.

In FY 2014, the number of radiation workers employed at other nuclear facilities such as nuclear power reactor facilities at the research and development stage (2 facilities), nuclear reactor facilities for test and research (12 facilities), fuel fabrication facilities (6 facilities), fuel reprocessing facilities (2 facilities), radioactive waste burial and waste management facilities (4 facilities), and research reactors and nuclear fuel handling facilities belonging to universities, research institutes or companies (15 facilities), was about 18 500 persons [53]. The average effective dose for workers at these facilities was <0.1 mSv. The highest dose for workers was 4.7 mSv, at the fuel reprocessing facilities. In FY 2009, before the 1F-NPP accident, the average effective dose for workers at these facilities was <0.2 mSv and the maximum dose was 6.2 mSv at fuel reprocessing facilities. The effective dose for most workers did not exceed 5 mSv/year. Thus, the equivalent dose to the lens for workers during normal operation at these facilities would also be sufficiently lower than the new ICRP dose limit to the lens. There have been few tasks for which eye dosimetry was explicitly required, with a few exceptions at the Japan Atomic Energy Agency (JAEA). One such exception involved repair operations where only the upper bodies of workers were exposed in a partially opened hot cell. In that case, additional dosemeters were issued to clip on the workers' caps (i.e., at their foreheads); the average ratio of the forehead/torso γ dose was ∼2 [58]. Another such work situation is the glovebox operation in the fabrication process of mixed oxide fuel (containing 20%–30% plutonium by weight) for use in fast breeder reactors. The wearing of lead protective aprons is standard practice here, thus dose-rate gradients requiring workers to be monitored with a pair of dosemeters, so-called double dosimetry. The eye dose is computed from the γ H_p(10) of one dosemeter outside the apron (around the collar) plus the neutron H_p(10) of the other dosemeter under the apron. Eye exposure data from the past 10 years indicate that no workers have exceeded the annual dose of 20 mSv, with a maximum of 18.9 mSv recorded in 2008 [59].

Works for recovery and reactor decommissioning were conducted after the 1F-NPP accident on 11 March 2011. Currently, around 10 000 workers per month conduct decommissioning tasks in the NPP [60]. Their exposed doses were high during the early phase of the accident, when the recovery operation was conducted. Internal exposure doses for some workers were especially high, mainly due to intake of ¹³¹I released from the reactors. The equivalent dose to the thyroid for two workers was distributed in the 10 000–15 000 mSv range, and these doses were the maximum [61].

The effective dose from external exposure was 199.42 mSv at maximum [62]. After the completion of Step 2* on 16 December 2011, the workers whose effective dose exceeded 100 mSv were not engaged in radiation work until 31 March 2016, following the official notice by the Ministry of Health, Labour and Welfare (MHLW), excluding some exceptional cases. Those exceptional workers were not engaged in radiation work from 1 May 2012 to 31 March 2016.

The effective dose limit under Japanese laws and ordinances for the emergency work, which was originally 100 mSv, was temporarily increased to 250 mSv, from 14 March to 16 December 2011 [63–65].

In Japan, the effective dose limit for emergency work was 100 mSv originally. The Japanese government enforced the laws and ordinances providing a higher limit of 250 mSv for an exceptional emergency, such as the 1F-NPP accident, on 1 April 2016 [66].

Footnote: * Step 2 was a process to attain stable cold shutdown conditions of the nuclear reactors as described on the 'Roadmap towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station' (Revised Edition of 17 October 2011 by the Government/Tokyo Electric Power Company Joint Office, Nuclear Emergency Response Headquarters).

Ideally, equivalent dose to the lens should be evaluated primarily using H_p(3), but it is evaluated using H_p(10) or H_p(0.07), whichever is the more appropriate value considering type and energy of radiation, according to an official notice of MHLW in Japan [67].

Radiation in the working environment at the 1F-NPP in the early phase of the accident was mainly γ-rays [68, 69]. Additionally, full-face masks were obligatory in all on-site areas in the 1F-NPP except for the inside of the Seismic Isolated Building. H_p(10) measured with alarm pocket dosemeters or other methods was therefore used for evaluating equivalent dose to the lens in early phase of the accident, until the establishment of 'β ray areas' (section 2.5.3).

Accordingly, maximum equivalent dose to the lens during the one-year period after the accident was the same as the effective dose from external exposure, which was 199.42 mSv [70].

Water treatment plants to decontaminate and desalinate the contaminated water pooled in the basements of the turbine buildings were built, and started operation. Cesium absorption equipment was set in these water treatment plants, and ⁹⁰Sr/⁹⁰Y is the main source of radiation in the contaminated water after cesium removal.

In order to handle this situation, ⁹⁰Sr/⁹⁰Y dominant areas with an H_p(0.07)/H_p(10) ratio over 4 or suspected areas were defined as 'β ray areas' [71], and the management in the areas started in August 2011.

The effective dose limit was increased to 250 mSv, among the increased dose limits of occupational exposure in emergency exposure situations in 1F-NPP on 14 March 2011. Considering such changes in dose limits, the criteria of the β ray areas were set with the aim that the dose limit for the hands and feet (skin) would not be exceeded by work in the environment where ⁹⁰Sr/⁹⁰Y was the main radiation source.

Basic personal protection equipment (PPE) for work handling contaminated water in β ray areas is a full-face mask, nonwoven fabric coverall, an anorak, cotton gloves, double rubber gloves, socks, and long boots. In some cases, such as in demolition work of tanks with high β contamination, special rubber gloves or other special equipment are used. Wrist badges or ring badges should be used for exposure on extremities in addition to active personal dosemeters (alarm pocket dosemeters).

There was a case of detection of a high β dose. On 3 February 2012, oozing water was found at the joint of a concentrated water storage tank. This tank formed a part of a desalination system (RO). A high dose was confirmed locally in a gap between a concrete base and a flange of a tank, just below the coupling, with oozing. On the surface at this point, 22 mSv h⁻¹ as H_p(10) and 2000 mSv h⁻¹ as H_p(0.07) were detected [72].

In order to reduce dose inside the 1F-NPP and suppress rainwater penetrating underground, pavement coating of a wide area is underway. The coating of the land surface, other than the surrounding areas of Units 1 to 4, was conducted until March 2016 [73]. As a result of these dose reduction efforts, H_p(10) in the 1F-NPP was ≤5 μSv h⁻¹ by the end of March 2016, except for the area surrounding the reactor buildings of Units 1 to 4 [74]. As the next step, since March 2016 the highly contaminated area surrounding the reactor buildings of Units 1 to 4 has been separated from other areas. The new operation rules using PPE according to areas with each contamination level were implemented after 8 March 2016, with the establishment of PPE exchange facilities [60, 75]. Under this new operation, PPE in the low contamination area was changed from coveralls with a nonwoven fabric to a normal work uniform or 1F-site-only clothes.

For lens dose assessment in 1F-NPP, H_p(0.07) on personal dosemeters attached to the chest (or abdomen, for women) is used without any conversion. In this case, the shielding by polycarbonate full-face masks is not taken into account, although all workers in high β ray areas use full-face masks. Thus, the dose assessment is considerably conservative compared with the true equivalent dose to the lens, and these conservative data have been recorded.

In the progress of the recovery operation, treatment of RO concentrated salt water with a high component of sea water, which is highly contaminated disposal water with a high concentration of strontium, was completed on 27 May 2015, except for residual water in the bottom of the tanks [76]. With this operation, the highly contaminated water with β sources of ⁹⁰Sr/⁹⁰Y mostly disappeared from the tanks. Additionally, absorption of cesium and strontium by dedicated equipment is continuing. The water processed by this equipment is called 'strontium-removed water', which is being re-purified by multi-nuclide removal equipment (ALPS) [77]. Thus, workplaces with ⁹⁰Sr/⁹⁰Y as the main radiation source have been reduced since 27 May 2015.

The MHLW released its 'Report of the Expert Meeting on Epidemiological Studies Targeting Emergency Workers at the TEPCO Fukushima Daiichi Nuclear Power Plant' on 4 June 2014, which recommended including cataract and other non-cancerous diseases in future epidemiological studies whenever possible [78].

Considering the long-lasting reactor decommissioning work and the MHLW policy on epidemiological studies, establishment and operation of a more accurate method for dose assessment of the lens is expected. This dose assessment should appropriately include the shielding effects of full-face masks in addition to other relevant factors and should be an accurate assessment with minimal conservatism.