Occupational eye lens dose in endoscopic retrograde cholangiopancreatography using a dedicated eye lens dosimeter

The International Commission on Radiological Protection (ICRP) recommended in 2011 that the dose limit of the eye lens should not exceed 20 mSv on average for 5 years and 50 mSv per year [1]. This is based on the fact that the threshold dose for cataract is estimated at 0.5 Gy, based on recent epidemiological research and other results [2, 3]. However, during interventional radiology (IR) and interventional cardiology procedures, physicians may be injured owing to prolonged exposure to x-ray radiation [3–7]. The radiography and fluoroscopy system with an x-ray tube above the patient table causes a higher amount of exposure to the upper body of the physician [8, 9]. In Japan, most hospitals use an overcouch x-ray tube system because upper gastrointestinal series using barium is still performed today. The advantage of the overcouch x-ray tube system is that the flat panel detector is positioned under the table, so there is no contact between the patient and the system. Since the x-ray tube is above the table and there are many scattered rays around the upper body, there is concern about the exposure dose to the eye lens. Moreover, this system with an x-ray tube is often used in nonvascular fluoroscopy-guided interventional procedures. Since endoscopic retrograde cholangiopancreatography (ERCP) is often diagnosed and treated, x-ray fluoroscopy time tends to be longer. Therefore, the actual exposure of medical staff during ERCP has been reported in many previous studies [10, 11]. In particular, the overcouch x-ray system may result in a higher occupational lens dose exposure than the undercouch x-ray system [12]. The ICRP operational quantity H_p(3) is used to monitor doses to the lens of the eye [13]. In ICRP Publication 139, the undercouch x-ray system is recommended for use as it can reduce the physician's eye lens dose by 2–27 times compared to the overcouch x-ray system [14]. The eye dosimeter is a useful solution for estimating H_p(3), and it can be worn behind lead glasses. In addition, this dosimeter has good fundamental characteristics (batch uniformity, dose linearity, energy dependence, and angular dependence) in the diagnostic x-ray energy domain [15]. This dosimeter can evaluate the exposure dose to the lens of the operator in IR based on H_p(3) and can control the threshold dose of cataract in detail [16–18]. Furthermore, scattered x-rays in IR can be reduced using radiological protection equipment, such as lead (Pb) glasses and radiation-protective curtains [19–22]. Typical ceiling-suspended lead screens have only one shielding direction and do not provide any protection if the user is outside the shielded field. Radiation-protective curtains can shield scattered radiation in four directions, thus providing shielding for the entire field. Moreover, since the protection provided by lead glasses depends on the angle at which scatter from the patient is incident on the head, the x-ray beams incident from the side and below the level of the head should be taken into account during assessment of the eye lens dose. The size of the lenses, use of side shields for glasses with flat lenses, and closeness of the fit to the facial contours are all important in determining the extent of protection provided [14].

In this study, we aimed to measure the lens-equivalent dose in nonvascular fluoroscopy using the radiography and fluoroscopy system with the x-ray tube above the patient table, which is considered to be exposed to the lenses in detail, using 3 mm equivalent dosimeters. We also examined whether the use of radiological protection equipment such as Pb glasses and radiation-protective curtains on the x-ray equipment reduced physicians' exposure to scattered x-rays.

2.1. Experimental design

2.2. Scattered x-ray dose at the physician's standing position

2.3. Measurement of lens-equivalent dose using dosimeters

The physician's lens-equivalent dose was measured using the eye dosimeter (DOSIRIS) and the neck dosimeter (glass badge). DOSIRIS was attached to the eyes of the head phantom, and the neck dosimeter was attached to the left side of the neck (figure 1(A)). DOSIRIS measurements were compared between the left and right sides. In addition, the extent to which the measured values for DOSIRIS and the neck dosimeter differed was verified. The direction of the head phantom was tilted 45° toward the head side with respect to the patient table. The position of the lens was similar to the previous experiment (160 cm from the floor), whereas the radiography and fluoroscopy system was set to a height of 85 cm from the floor to the bed, an SID of 114 cm, and an FOV of 28 × 28 cm. This position is the home position of this system, and the ERCP procedure is usually performed at this position at our group hospital. Also, the x-ray fluoroscopy time during ERCP at this hospital averaged at 32.8 min in 22 cases. ERCP involves diagnosis, followed by therapy, such as endoscopic nasobiliary drainage and endoscopic lithotripsy. Table 1 shows the exposure conditions of the fluoroscopy program used in this study. The examination model was ERCP, which tends to increase the x-ray irradiation time to develop into treatment. The whole-body phantom was irradiated with x-rays for 30 min. The measurements were repeated five times. New dosimeters were used each time, and the average was calculated.

Zoom In Zoom Out Reset image size

Table 1. Exposure conditions of the fluoroscopy program.

KAP (Gy cm2)	237
ESD (mGy)	547
Voltage (kV)	105
Current (mA)	2.3
Exposure time (min)	30
Total tube current–time product (mAs)	4140
SID (cm)	115
FOV (cm)	28 × 28
Table height (cm)	85

KAP: kerma-area product.ESD: entrance surface dose.SID: source-to-image distance.FOV: field of view.

2.4. Reduction effect using radiation-protection devices

Two types of Pb glasses (PT-53 0.75 mm Pb; Maeda and HF-350 0.07 mm Pb; Hoshina) were used to evaluate the effect of radiological protection equipment on eye exposure. Figure 2 shows the shape of each of the Pb glasses. DOSIRIS was set inside the Pb glasses against the left and right eyes to measure the dose to the eyes (figure 1(B)). Furthermore, the effect of radiation-protective curtains (NP 0.25 mm Pb), which could reduce scattered radiation from the phantom, was also evaluated. The curtains were attached to the x-ray tube (as shown in figure 1(C)), and the dose to the eyes was measured using the same method. Since the radiation-protective curtains cover only the area just below the x-ray tube, they do not interfere with the ERCP procedure. Furthermore, the curtains have a slit at the bottom; thus, there is little contact between the curtains and the patient.

Zoom In Zoom Out Reset image size

2.5. Statistical analysis

3.1. Scattered x-ray doses at the physician's standing position

Figure 3 shows the scattered x-ray dose in the standing position of the physician. The scattered x-ray dose showed a low value of 12.34 μGy min⁻¹ at a height of 80 cm from the floor and linearly increased from 90 to 130 cm. Scattered x-ray doses peaked at 140 cm and then linearly decreased.

Zoom In Zoom Out Reset image size

3.2. Measurement of lens-equivalent doses using dosimeters

Figure 4 shows the average value of the physicians' eye dose when the whole-body phantom was irradiated with x-rays for 30 min during ERCP using the overcouch x-ray tube system. The value of each dosimeter showed the highest value for the neck badge [H_p(0.07)]. The dose measured using DOSIRIS for the left eye was not significantly different from the measurements made with the neck dosimeter. However, the dose to the right eye was substantially lower.

Zoom In Zoom Out Reset image size

3.3. Protective effects using radiation-protection devices

In this study, there was no significant difference in the measured values when the exposure to the lens was measured using a lens dosimeter or a neck personal dosimeter. It has been shown that radiation-protective devices such as radiation-protective lead glasses and scattered radiation-protective curtains can significantly reduce IR physicians' exposure.

For the examination model as ERCP, the scattered x-ray dose at the position of the physician was the maximum at a height of 140 cm from the floor, probably influenced by backscattered x-rays from the whole-body phantom. It is thought that when a whole-body phantom with a body thickness of 20 cm is irradiated with x-rays, the ratio of back scattering is higher [23]. Scattered x-rays are also generated from the acryl plate of the irradiation window of the x-ray tube [24]. This height corresponds to the thyroid gland because the average height of Japanese men is 171 cm [25]. Our results based on ERCP simulation using the overcouch x-ray tube system may indicate the risk of cataract since the dose to the physician's eyes during the irradiation to the whole-body phantom for 30 min ranged from 1.5 to 3.7 mSv. O'Connor et al state that the mean equivalent dose to the lens of the eye of a gastroenterologist is 0.01 mSv per ERCP procedure with an undercouch x-ray tube and 0.09 mSv per ERCP procedure with an overcouch x-ray tube. However, the fluoroscopy time for this study was 225 s and 87 s in the two hospitals [10]. Zagorska et al report that the fluoroscopy time varied from 1.0 min to 28.8 min owing to the different complexities of ERCP procedures [11]. This means that our study estimated the maximum dose to the physician's eyes during an ERCP procedure. However, in our study, kerma-area product was higher than the national diagnostic reference levels for ERCP in Japan, which may have increased the physician's lens dose. The dose to the left eye using the overcouch x-ray tube system (3.7 mSv) corresponded to 7.4% of the maximum annual limit proposed by ICRP (50 mSv); therefore, the total dose can reach the proposed dose if ERCP is performed 14 or more times a year. The reason the dose to the left eye was high could be explained as follows: the left eye was closer to the irradiation centre than the right eye in the phantom head setting with 45° rotation in our study (figure 2). In our study, the lens-equivalent dose was measured using Pb glasses of two kinds of lead equivalent. Pb glasses with a small lead equivalent are advantageous because of their light weight. Originally, the radiation-scattering reduction rate was expected to be higher for Pb glasses with a larger lead equivalence [26]. However, in this study, 0.07 mm Pb glasses showed higher scattered ray reduction rates than 0.75 mm Pb glasses in both eyes. This is related to the difference in the shape of the glasses. Dorey et al proposed an important observation, i.e. the increased dose rate the clinician's eyes receive despite wearing Pb glasses, as the angle of gaze moves to 45° and 90° from 0° toward the scattering source. The gap between Pb glasses and the skin becomes wider as the angle of gaze increases, allowing more scattered x-rays to enter the eye [27]. The lens and frame of 0.75 mm Pb glasses are flat and straight; therefore, the gap between these glasses and face/eyes is wider than that between the face/eyes and 0.07 mm Pb glasses, which are curved to fit the face, resulting in less gas between the glasses and face/eyes. Our study indicated that the shape of the frame and lens is also critical in reducing the scattered radiation to the eyes, although lead thickness is important for providing better protection. The scattering x-ray reduction rate reduced to 33.3%–80.0% by attaching the Pb glasses in our study. The results of this study are similar to those of previous studies [19, 28, 29]. Moreover, 96.8% of the scattered x-rays were reduced when the radiation-protective curtains were attached to the x-ray tube. Our study results were similar to those of Minami et al, who reported similar radiation reduction effects using radiation-protective curtains [30, 31]. In fact, radiation-protective curtains were not particularly obstacles in helping physicians perform ERCP examinations, which do not require sterile conditions. Although radiation-protective curtains provide higher protection against scattered x-rays than Pb glasses, they are not very popular because they are expensive.

In this study, the physician's lens-equivalent dose during ERCP was estimated using the overcouch-type x-ray television system, which is said to project more x-ray doses to the physician's upper body. As a result, we clarified that the lens-equivalent dose during each ERCP examination shows a very high proportion to the annual lens-equivalent dose recommended by the IRCP, and we also warned against cataract risk. As a precautionary measure, the use of Pb glasses and especially radiation-protective curtains is highly effective in reducing scattered x-rays; thus, we recommend using these. In addition, since the undercouch x-ray tube system has been shown to reduce the exposure dose to the upper body of the physician, the introduction of these systems is recommended in the future.

Our study had several limitations. First, during an actual examination, the physician needs to turn their face in different directions; however, these effects were not included in this study because the physician will move their face but should, in fact, be looking at the TV monitor during x-ray irradiation. Second, although the size of the x-ray irradiation field changes depending on the situation, this measurement was performed with only a 28 × 28 cm FOV. In the future, we will attempt to verify the effect of the angle of the physician's face on the reduction of the lens dose.

Our study showed that the lens-equivalent dose reduced by up to 80.0% when using Pb glasses and by 96.8% when using radiation-protective curtains. The eyewear that is larger and fitted more closely to the face provided a better protection effect even with a low lead equivalence, demonstrating that the shape of eyewear is important for protective function.

ERCP with the overcouch x-ray tube system is associated with the risk of cataract when performed several times without the use of radiation-protective equipment.

Thus, we recommend using Pb glasses, especially radiation-protective curtains, owing to their extremely high scattered radiation reduction effect.

We are especially grateful to Professor Toshizo Katsuda for his continuous support and thoughtful guidance throughout the program. This study received funding from the Japan Society for the Promotion of Science KAKENHI (Grant Number JP 17K09142).

Institutional Review Board approval was not required because this study was a phantom study.