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Patient and staff exposure during endoscopic retrograde cholangiopancreatography

Departments of ¹ Radiology and Medical Imaging and ² Gastro-Enterology, Free University Hospital Brussels (AZ-VUB), Laarbeeklaan 101, B-1090 Brussels, Belgium

	Abstract

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Despite a number of efforts being put into the radiological protection of both patient and staff during interventional radiological (IR) procedures during recent years, information about radiation exposure during endoscopic retrograde cholangiopancreatography (ERCP) procedures remains scarce. The purpose of this study was to estimate both patient and staff radiation doses during therapeutic ERCP procedures by direct measurement and to compare these results with data from other IR procedures. For 54 patients, effective dose and skin dose were estimated by measuring the dose–area product. For staff, entrance surface doses to the lens of the eye, thyroid and hands were estimated by thermoluminescent dosemeters. A median effective dose of 7.3 mSv and a median entrance surface dose of 271 mGy per procedure were estimated for patients. The gastroenterologist received a median dose of 0.34 mGy to the lens of the eye, 0.30 mGy to the skin at the level of the thyroid and 0.44 mGy to the skin of the hands, per procedure. When comparing the dosimetric quantities presented in this study with data from other IR procedures, it is clear that patient skin doses and doses to staff are high owing to the use of inappropriate X-ray equipment. ERCP requires the same radiation protection practice as all IR procedures. It should be consistently included in future multicentre IR patient and staff dose survey studies at national or international level.

	Introduction

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Endoscopic retrograde cholangiopancreatography (ERCP) was first introduced in 1969 [1]. It is a common interventional procedure that examines the duodenum, papilla of Vater, bile ducts, gallbladder and pancreatic duct. Although ERCP was introduced as a purely diagnostic procedure, technical advances in instrumentation during the last 2 decades have added a therapeutic dimension.

During the performance of ERCP, a high amount of X-ray fluoroscopy and digital radiographs are performed, making it an interventional radiological (IR) procedure [2]. It is well known that some of the highest doses to patients from diagnostic medical X-rays other than CT arise from IR procedures [3]. One of the key issues emphasised during the recent International Conference on Radiological Protection of Patients in Diagnostic and Interventional Radiology, organized by the International Atomic Energy Agency, was that there is a need for radiation risk evaluation and reference doses in IR to minimize somatic risks and to avoid deterministic injuries [4]. Despite the fact that, during recent years, a number of efforts have been put into the protection of both patients and staff, information about radiation exposure during ERCP remains scarce. Published work on radiation protection in IR is mainly focussed on various angiographic and cardiological procedures [5–16]. Few international papers have been published that evaluate radiation exposure to patients [7, 17–20] and staff [21–23] during ERCP.

The purpose of this study is to estimate patient dose, with respect to Article 8 of the Euratom 97/43 directive [24], and to estimate the dose to the lens of the eye, thyroid and hands of staff. For the patient, the effective dose E, as an index of the risk for stochastic effects [25], and the entrance surface dose (ESD) are estimated by measuring the dose–area product (DAP). Doses to critical organs located in the area of exposure are also estimated. The ESD is estimated in order to assess the possibility of skin dose exceeding the threshold for deterministic effects. Concerning staff, the ESD to the eyes, neck and hands are estimated by using thermoluminescent dosemeters (TLDs).

It should be noted that, as in many other IR procedures, ERCP is not performed by a radiologist but by a gastroenterologist (GE). Although it is a requirement of the Euratom 97/43 directive [24] that staff performing practical aspects of a medical exposure should have received adequate training in radiation protection, a GE has not had in-depth training in radiation management using diverse forms of fluoroscopic equipment, nor in the potential harmful effects to patients and staff.

	Materials and methods

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During a 4-month period (May–September 2000), patient and staff doses were evaluated from 54 therapeutic ERCP examinations; 23 male and 31 female patients. The age of the male group ranged between 43 years and 77 years, with a mean age of 63 years. The age of the female group ranged between 40 years and 85 years, with a mean age of 70 years.

Radiographic unit
The ERCP procedures were performed on a General Electric Prestilix 1600 DRS radiographic unit (General Electric Medical Systems, Milwaukee, WI), equipped with an overcouch tube (model RSN722; General Electric Medical Systems, Milwaukee, WI) (total filtration=3.2 mm Al equivalent) and a high frequency generator. Input air kerma rates for each image intensifier field size selection, in standard and high detail mode, are listed in Table 1. The input air kerma per image for the digital radiographs were 2.34 µGy (field size 15 cm), 1.59 µGy (23 cm) and 0.81 µGy (38 cm). Measurements were performed on top of the image intensifier with a 150 cm³ ionization chamber (Keithley, Cleveland, OH) and 2 mm additional copper filtration.

Almost every ERCP is performed under local sedation. Full anaesthesia is used in only a few cases. No such cases are included in this study. Endoscopy is always performed with the patient in the recumbent left lateral position. During the procedure, fluoroscopic screening and digital radiographs of the hepatic–biliary area are taken with a varying tube potential between 100 kVp and 125 kVp, depending on patient size. All exposures are performed in the automatic brightness control (ABC) mode. The procedure has a fixed set-up, the patient stays in the same position and the same anatomical area is exposed during the entire examination.

An overview of patient and staff positions is shown in Figure 1. During the procedure, the GE, who is assisted by a nurse (N1), stands with his left side next to the patient and manipulates the duodenoscope. A second nurse (N2) holds the patient's head in order to position the duodenoscope. With respect to dosimetry, it is important to note that both nurses do not remain in the same position all the time. They often leave their position for short durations to operate monitoring equipment or to reach other medical equipment. All members of staff are classified as occupationally exposed workers and are, in consequence, already monitored by a regulatory dosemeter worn at chest level. They all wear wrap around aprons during procedures, providing a protection of 0.50 mm lead equivalence at the front of the body and 0.25 mm lead equivalence at both sides and the back of the body.

View larger version (35K): [in this window] [in a new window]   Figure 1. Staff positions relative to the patient and X-ray tube during a typical endoscopic retrograde cholangiopancreatography procedure. Distance variations are marked in centimetres. G.E., gastroenterologist; N.1., assisting nurse; N.2., second nurse.

Estimation of patient dose
In the field of interventional radiology, where both fluoroscopy and digital radiography are used at various kilovoltages and various X-ray projections are assessed, DAP as a measure of radiation dose to the patient is widely applied [3]. DAP is measured by a transmission ionization chamber (model TV57523-1; PTW, Freiburg, Germany) mounted on the collimator box of the X-ray tube and connected to a Diamentor M2 (PTW, Freiburg, Germany) read-out unit. The DAP equipment was initially calibrated by the Physikalisch-Technische Bundesanstalt (Braunschweig, Germany) and is verified every 6 months in our department.

During examination, DAP values were determined for each fraction of fluoroscopic exposure and digital radiograph as a function of the selected kilovoltage and the anatomical projection. Diasoft software (PTW, Freiburg, Germany) was used to register DAP values. Effective dose and organ doses were estimated separately for each patient by using recorded DAP values in combination with effective dose conversion factors derived from Monte Carlo computer calculations for male and female mathematical anthropomorphic phantoms [26]. These factors are expressed as a function of beam quality, applied kilovoltage and radiographic projection. Since the radiographic projection that was used for a typical ERCP is not specifically defined by Hart et al [26], it was approximated by a combination of the stomach left lateral and the duodenum left posterior oblique projections. An overview of a typical ERCP procedure in terms of irradiation site, radiographic projection (as used for the dose calculation), type of exposure (fluoroscopy or digital images), image intensifier field size selection, tube potential, air kerma mode for fluoroscopy and number of digital radiographs is shown in Table 2. For a typical ERCP procedure, four DAP values were determined; one for each kilovoltage at each projection. Fractionation of the DAP data was carried out during the procedure with the Diasoft software (PTW, Freiburg, Germany). Dose was calculated for each patient from each contributing DAP fraction and the summation per organ was made, giving the total effective dose and organ doses from that procedure.

Estimation of staff dose
As a classified worker, each member of staff is already monitored by a regulatory dosemeter worn at chest level under the apron. Since we were interested in the ESDs per procedure on parts of the body that are not protected by the apron, additional measurements were performed. ESDs at three different positions on the body were estimated by using a batch of 50 TLDs TLD-100H (LiF: Mg, Cu, P; 3.2 mm x 3.2 mm x 0.5 mm; Harshaw-Bicron, Wermelskirchen, Germany). For each position, three TLD chips were inserted in a thin polyethylene plastic bag and attached to the skin by adhesive tape. Three bags with a total of nine TLD chips were used for each staff member per ERCP procedure. One bag was attached to the forehead, one to the skin of the neck at the level of the thyroid and one to the dorsal surface of the hand closest to the patient (left hand for the GE and N1 and right hand for the N2). TLDs were calibrated and processed as previously described.

	Results

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Estimation of patient dose
Phantom measurements show that for the range of ERCP exposure settings (image intensifier field size 38 cm at approximately 90 kVp, field size 23 cm at approximately 105 kVp and field size 15 cm at approximately 125 kVp), the ESD and DAP exhibit an almost perfect linear relationship (r=0.99, p<0.001). Data are shown in Figure 2 (error bars represent 95% confidence intervals). Uncertainties are estimated to be 6% for the DAP reading (including DAP meter calibration and DAP meter response) and 8% for the ESD reading (including TLD calibration, TLD response and TLD positioning). A DAP (Gycm²) to ESD (Gy) conversion coefficient of 6.96 x 10^-3 cm^-2 is determined for the estimation of the ESD to patients. As shown in Figure 2, the coefficient is independent of image intensifier field size selection. This is owing to the ABC system of the radiographic equipment, which adjusts exposure factors in order to deliver a constant image intensifier output signal. A smaller image intensifier field size selection automatically implies a smaller field of view, which is compensated by the ABC system increasing exposure factors. As a result, the DAP value stays constant for the three image intensifier field size selections. This makes the established DAP to ESD coefficient independent of the image intensifier field size selection on condition that no additional collimation is used and the ABC system works properly (which is the case for this study). However, caution is necessary when using this conversion coefficient. It was established using a fixed distance between tube and phantom and, despite the fact that phantom and patients were exposed in the same geometrical condition, the FSD will vary slightly depending on the thickness of the patient. A variation in FSD will strongly affect the air kerma at the skin surface, owing to the inverse square law, and thus the ESD. Because of this uncertainty, patients with body weights below 60 kg and above 80 kg were excluded from this study and a 10% variation in FSD was accepted, resulting in a 20% variation of the air kerma at the patient's skin surface (see Accuracy of the estimations).

View larger version (12K): [in this window] [in a new window]   Figure 2. Entrance surface dose (ESD) measured on the surface ofan Alderson-rando phantom (Phantom Laboratory, Salem, NY) as a function of dose–area product (DAP) for field size selections 38 cm (), 23 cm () and 15 cm () in a typical endoscopic retrograde cholangiopancreatographyprocedure set-up.

Estimation of staff dose
An overview of the ESD data to the eyes, neck and hand of the GE, N1 and N2 per ERCP procedure is listed in Table 4.

	Discussion

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Patient dose
From the data in Table 3, it can be seen that exposure to the patient shows large variation. This can easily be understood considering the influence of patient size on tube loading and the variation in the assessed screening time according to the conduct of the procedure and the patient's condition. A median effective dose of 7.3 mSv and a median skin dose of 271 mGy were estimated for the group of 54 patients.

In all cases, ESD remained well below the threshold single fraction dose of 2 Gy for transient erythema [28]. The 3rd quartile value of the group was 420 mGy and the maximum measured value was 1180 mGy. Nevertheless, care should be taken since some "high dose" patients can undergo two or more ERCP procedures consecutively. In normal circumstances, the time period between consecutive ERCPs of a patient is 48 h.

Table 5 compares the effective dose and ESD estimated in this study with data from other common IR procedures previously published in the literature, with special interest to the biliary–hepatic region. Almost all data are retrieved from McParland [5, 6] and Ruiz-Cruces et al [7]. All values presented in Table 5 are median values unless stated differently. A comparison of effective dose from different procedures is made since this property is an index of the risk of stochastic effects. DAP on its own is useful when comparing separate procedures of a similar type e.g. ERCP in this case, but since this value is independent of the radiosensitivity of the screened anatomical region, effective dose is a better index when comparing different types of radiological procedures [5].

The effective dose to the patient from an ERCP procedure is of the same order of magnitude as the dose from other IR procedures listed in Table 5. However, the ESD is relatively high owing to the clinical technique used and the limitations of the radiographic equipment (fixed overcouch tube). The area of irradiation remains fixed during the entire ERCP procedure, whereas during the other procedures listed in Table 5 the exposure can be distributed over a larger skin area, which reduces the maximum skin dose value to a specific point.

Staff dose
The data presented in Table 4 illustrate that the GE receives the highest dose to the eyes, thyroid and hands (all cases p<0.02, two-tailed Spearman rank correlation test) in relation to the nursing staff owing to proximity to the area of exposure. Doses to the two members of nursing staff are comparable. Distance to the patient of the N2, handling the patient's head, is larger than that of the N1. The latter, however, is partially shielded by the GE. Although dose to the hands is higher, dose to the lens of the eye will be the critical organ relative to regulatory dose limits. Considering the annual dose limit to the lens of the eye of 150 mSv [25] for a classified worker, and the 3rd quartile values presented in Table 4, a GE could annually perform approximately 225 ERCP procedures before reaching the dose limit. 3rd quartile values are used from a pragmatic radiation protection point of view. For N1 this value is approximately 440, and for N2 305. As the annual number of ERCP procedures performed in our hospital by one GE is approximately 120, it is very unlikely that this limit will be reached. Nevertheless, doses to staff can and should be decreased to a reasonably achievable level [24, 25].

Table 6 compares the doses to ERCP staff with those from other IR procedures published in the literature. All values are median values unless stated differently. Germanaud et al [21] also used a conventional overcouch tube to perform ERCP and assessed direct measurements of staff dose by thermoluminescent dosimetry. The median dose to the lens of the eye was found to be 0.13 mGy per procedure and the ESD at thyroid level was measured at 0.10 mGy. Although dose to staff was found to be lower in their study, the mean fluoroscopic screening time (6.8 min) was comparable with the mean screening time in our study. The difference in staff doses can be attributed to several factors: they used a lower kilovoltage (between 75 kV and 95 kV), which affects the amount and energy of scatter radiation and therefore dose; location of the GE; and the difference in dose rate during fluoroscopy and statistical variation. Cohen et al [22] estimated a much higher dose when performing indirect phantom measurements and assuming a fluoroscopic screening time of 20 min. Unfortunately, they did not specify the type of X-ray equipment used.

General discussion
This study demonstrates that DAP monitoring is an easy and useful tool for estimating patient effective dose and ESD in ERCP procedures. From the dosimetric quantities presented in this study, it is obvious that an ERCP procedure has the potential to cause high patient and staff doses and consequently requires attention regarding radiation protection. Whilst it is well known that an overcouch tube X-ray unit is not adequate for performing IR procedures [10], all of the studies consulted regarding ERCP [17–23] involved the use of this type of equipment (or did not specify which equipment type was used). This may indicate that ERCP procedures are often performed without attention to equipment and radiation protection in comparison with more common IR procedures such as angiographic and cardiological interventions. The reason for this lack of ERCP dedicated X-ray equipment is probably cost-benefit related.

It is clear that a correctly operated C-arm unit with the availability of pulsed fluoroscopy will dramatically reduce dose to both patients and staff. In a second stage, staff protection can be complemented by a thyroid collar shield, protective glasses for the GE and protective gloves for the N2. Secondary radiation shielding can also be very effective in reducing staff exposure. Chen et al [23] showed, in a 20 patient ERCP study, that a 0.5 mm lead equivalent acrylic shield reduced staff exposure by a factor of 11. The shield did not interfere with performance of the examination.

Another important factor affecting patient and staff exposure is communication between the radiographer and the GE. The radiographer should only perform fluoroscopic screening as required by the GE, and the latter should indicate clearly when no more screening is needed. From our experience it is clear that a relatively large amount of unnecessary exposure is delivered when no strict arrangements are made between staff members.

The authors advise that ERCP should be consistently included in future clinical multicentre IR patient and staff dose survey studies at national and international level.

	Acknowledgments

 
The authors wish to thank W Vroonen, RT, M Freson, MD, D Urbain, MD and the nursing staff of the gastroenterology department for their cooperation in obtaining measurements, as well as W Wijnen for his help in editing the manuscript.

Received for publication May 24, 2001. Accepted for publication November 29, 2001.

	References

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