1 INTRODUCTION

In April 2011, the International Commission on Radiological Protection (ICRP) Statement on Tissue Reactions (Seoul Statement), reduced the threshold dose for cataracts to 0.5 Gy and issued the following recommendation for the eye lens equivalent dose limit for occupational exposure in planned exposure situations: “for occupational exposure in planned exposure situations the Commission now recommends an equivalent dose limit for the lens of the eye of 20 mSv/year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv.1” In response to this ICRP recommendation, the relevant Japanese national policy, Ordinance on Prevention of Ionizing Radiation Hazards, and the lens equivalent dose limit was revised from 150 to 100 mSv over 5 years and 50 mSv/year (revised in April 2021).

The occupational dose to the lens of physicians involved in radiology procedure has been reported to be significant in interventional radiology (IVR) procedures for cerebrovascular2-5 and cardiovascular3, 6 medicine, tumors,3, 7 endoscopic retrograde cholangiopancreatography (ERCP),3, 8, 9 and orthopedic surgery.10 The lens equivalent dose limit is considered to have exceeded when cardiologists and gastroenterologists perform radiology procedure without radiation protection for the lens of the eye.3, 6, 9, 11 For this reason, the International Atomic Energy Agency disseminates information regarding the possibility of reducing exposure using lead glasses and ceiling-mounted radiation shielding screens and educates the workers on the importance of eye lens protection.12

ICRP reported that many physicians who perform radiology procedures have inadequate radiation protection.13, 14 Although the wear rate of lead aprons and neck guards by physicians performing radiology procedures is higher than 90%,15, 16 the wear rate of lead glasses is 30%–52%.3, 15-18 Moreover, despite the ability of lead-containing ceiling-mounted radiation shielding screens to reduce eye lens exposure by over 70%,19-22 these screens are not always used appropriately in actual medical procedure,17 putting physicians at risk of receiving high radiation doses to the lens of the eye.

In this multicenter study, by applying basic principles of occupational health, we proposed “multiple radiation protection” measures that did not place an undue physical burden on physicians and were less expensive for medical facilities and estimated the effectiveness of these measures. In addition, the occupational dose to the lens of the eye for physicians was measured on a case-by-case basis, and the potential for compliance with the new-equivalent dose limits to the lens of the eye (ICRP: average annual limit, 20 mSv/year over 5 years) was assessed.

2 METHODS 2.1 Participants for measurement

Between April 2019 and July 2019, 15 physicians engaged in radiology procedure (angiography, non-angiography, or IVR procedure) at 15 medical facilities in Japan were nominated by their respective societies (Japanese Society of Radiology and the Japanese Society of Interventional Radiology, Japanese Orthopedic Association, Japanese Society of Gastroenterology, and Japanese Society of Neuroendovascular Therapy). Eye lens doses (3-mm dose equivalent, i.e., Hp(3)) were measured for each participant when they performed radiology procedure using conventional methods before implementing radio-protective measures (before radiation protection measures, Table 1) and after the implementation of radio-protective measures (after radiation protection measures, Table 1), taking into account the facility environment and the procedures in place at each medical facility. We confirmed the doctor's radiation protection method from the pre-questionnaire and the photographs during the procedure. The personal dose values for the past 3 years and the number of procedures performed over the past year were also investigated. Since there was no evaluation of 3-mm dose equivalent at that time, the personal dose values were defined as the 70-µm dose equivalent of the skin or the 1-cm dose equivalent of the effective dose, whichever is larger, as the eye lens dose.

TABLE 1. Radiation protection status before and after the radiation protection measures Physician Measures before/after Simultaneous irradiation Pulse rate reduction Irradiation field IR Lead glasses Shielding screens Cu Cs R Evacuation from the room Cardiologist A Before ○ ○ ○a ●f ○ After ○ ○ ○ ○a ○f ○ Cardiologist B Before ○ ○ ○ ●g ○ After ○ ○ ○ ○a ○g ○ Cardiologist C Before ○ ○ ○ ○b ○ After ○ ○ ○ ○c ○g ○ Neurosurgery D Before ○d ○ After ○ ○ ○ ○d ○g ○ Neurosurgery E Before ○ ○a ●g ○ ○ After ○ ○ ○ ○a ○g ○ ○ Gastroenterologist F Before ○ ○a ●h After ○ ○ ○a ○h Gastroenterologist G Before After ○a Gastroenterologist H Before ○a ○i After ○ ○e ○i Gastroenterologist I Before ●h After ○a ○h Gastroenterologist J Before ○h After ○a ○h Orthopedic Surgeon K Before ○ ○d ● After ○ ○ ○ ○d ○ ○ ○ Orthopedic Surgeon L Before After ○d Orthopedic Surgeon M Before After ○a Radiologist N Before ○ ○ ○ ○a ○g ○ ○ After ○ ○ ○ ○a ○g ○ ○ Radiologist O Before ○ ○ ○ ○a ○g ○ ○ After ○ ○ ○ ○a ○g ○ ○ Note ○ Radioprotective equipment was being used appropriately. ● Radioprotective equipment was used, but the method of use was inappropriate. Simultaneous irradiation = reduction of simultaneous frontal and lateral irradiation in fluoroscopy mode; Pulse rate reduction = appropriate selection/switching of fluoroscopy mode (changed from 15 to 7.5 pps); Irradiation field = restriction of irradiation field to target region; IR = dose reduction using iterative reconstruction (IR) during CT fluoroscopy; Shielding screens = ceiling-mounted radiation-shielding screens; Cu = under-bed protective curtains; Cs = scatter-radiation protection curtains; R = RADPAD®; Evacuation from the room = room evacuation during imaging mode. a Lead glasses (Panorama shield®; HF-400 ultra-light 0.07-mm Pb; Toray). b Lead glasses (HAGOROMO Face Guard FG06-110; 0.06-mm Pb; Maeda). c Lead glasses (CROSSLINK 0.75-mm Pb; Barrier technologies®). d Lead glasses (Panorama shield®; HF-350 ultra-light 0.07-mm Pb; Toray). e Lead glasses (ProTech eyewear PT-COMET 0.75-mm Pb; Maeda). f Ceiling-mounted radiation shielding screen 350 (0.5-mm Pb; Kenex). g Ceiling-mounted radiation shielding screen (MAVIG 0.5-mm Pb; MAVIG GmbH). h Scatter-radiation protection curtains (Scatter protection cloth NP, 0.125-mm Pb; Maeda). i Scatter-radiation-protection curtains (self-made scatter-protection clothing, 0.25-mm Pb). 2.2 X-ray equipment and radio-protective methods

Of the 15 participating medical institutions, six used biplane angiography, three used hybrid single-plane angiography combined with X-ray computed tomography (CT), and one used surgical X-ray fluoroscopy. The remaining five centers used X-ray fluoroscopy systems, of which four used over-table X-ray tube systems and one used an under-table X-ray tube system.

Radiation protection measures for physicians involved in radiology procedures included the use of reduction of simultaneous front-to-side irradiation during fluoroscopy, appropriate selection/switching of the fluoroscopy mode (switching from 15 pps to 7.5 pps), restriction of the irradiation field to the target range, dose reduction performed using iterative reconstruction (IR) in CT fluoroscopy in combination to avoid negatively influencing radiology procedure, lead glasses, ceiling-mounted radiation-shielding screens, scatter-radiation–shielding curtain for over-table X-ray tube systems, and under-bed protective curtains, RADPAD® (0.25 mmPb, Nippon Medical Readers Co., Ltd.), evacuation of the examination room during imaging. The lead glasses used were as follows: HF-400 (0.07-mm Pb-equivalent; Toray Medical Inc., n = 11), HF-350 (0.07-mm Pb-equivalent; Toray Medical Inc., n = 3), FG06-110 (0.06-mm Pb-equivalent; Maeda, n = 1), CROSSLINK (0.75-mm Pb; Barrier technologies®, n = 1) and PT-COMET (0.75-mm Pb-equivalent; Maeda, n = 1). The ceiling-mounted radiation-shielding screens used were as follows: ceiling-mounted radiation shield 350 (0.5-mm Pb-equivalent; Kenex, n = 1) and MAVIG (0.5-mm Pb-equivalent; MAVIG GmbH, n = 6). In addition, for the over-table X-ray tube systems, scatter protection was provided by NP cloth (0.125-mm Pb-equivalent; Maeda, n = 3) and facility-made scatter-protection curtain produced by each medical institution (0.25-mm Pb-equivalent, n = 1) (Table 1).

2.3 Details concerning radiation protection

We confirmed the physician's radiation protection method from the pre-questionnaire and the photographs during the procedure. We have proposed the main protection methods based on the current situation. (Based on the three principles of external exposure protection, installing a ceiling-mounted radiation shielding screen, reducing the pulse rate within the range that does not deteriorate the image quality, and if necessary, they were instructed to wear protective equipment.)

2.4 Method for measuring occupational dose to the lens of the eye

Physicians wore lead aprons and lead glasses. The Hp(3) to the lens of the eye was obtained from air kerma measurements obtained by radio-photoluminescence glass dosimeters23 (GD-352M; Chiyoda Technol) attached to the inner and exterior sides of the lead glasses. The GD-352M used in the measurements complied with the IEC62387 requirements for dosimetry systems with passive detectors and provided stable dose linearity in the low dose range (less than ±5.0% in the range of 0.01 to 50 mGy).23, 24 Before the start of this study, the coefficient of variation was confirmed to not exceed 3.0%. We modified a previously reported eye lens dosimeter clip25 component and attached it to the left and right sides of the lead glasses and placed one GD-352M unit each in fixed positions on the inner and outer sides of the lens (Figure 1A,B). For lead glasses where the eye lens dosimeter clips could not be used, GD-352M units were attached to the left and right sides of the lens using adhesive tape (Figure 1C,D).

Eye lens dosimeter clip position on the lead glasses used: (A) side view, (B) front view. Location of radio-photoluminescence glass dosimeters (RPLDs) on the lead glasses used (when tape is used): (C) side view, (D) inner side view. Four RPLDs are placed on the left and right sides of the lead glasses

After the measurements were completed, the radio-photoluminescence glass dosimeters were stored in a low-background area outside the radiation-controlled area and returned to the providing university by postal mail after the survey period. The data were then read and analyzed using a reading device (FGD-1000; Chiyoda Technol) installed at our institution.

The eye lens dose Hp(3) in this study was calculated from air kerma measurements obtained using the radio-photoluminescence glass dosimeters. Specifically, the air kerma to Hp(3) conversion coefficient K (Hp(3)/air kerma) on a cylindrical phantom (φ20 cm × 20 cm) was calculated using a Monte Carlo simulation from a previous report,26 and the conversion was performed according to Equation (1). In this study, the effective energy used in radiology procedure was assumed to be 50 keV, and 1.590 Sv/Gy was adopted for K (Hp(3)/air kerma): (1)where Hp(3) is the lens dose of the physicians eye per procedure (μSv), air kerma is the radio-photoluminescent glass dosimeter measured value (μGy), K is the air kerma to Hp(3) conversion coefficient (Hp(3)/air kerma) (Sv/Gy).

For each physician, the Hp(3) values measured before the radiation protection measures were compared between the left and right eyes, and the value indicating a greater dose was recorded as the Hp(3) in this study. For evaluations after the radiation protection measures, the Hp(3) values for the dose on the same side as that measured before the measures were recorded.

2.5 Method for calculating Hp(3)rate to the lens of the eye To determine the lens dose of the eye per unit time, the Hp(3) values obtained in Equation (1) were divided by the fluoroscopy time of the procedure to obtain the eye lens dose rate per unit time (Hp(3)rate) according to Equation (2) below: