British Journal of Radiology (2006) 79, 483-486
© 2006 British Institute of Radiology
doi: 10.1259/bjr/67632946
Radiation risks for the radiologist performing transjugular intrahepatic portosystemic shunt (TIPS)
N Hidajat, MD,
P Wust, MD,
M Kreuschner, MD,
R Felix, MD and
R-J Schröder, MD
Department of Radiology, Charité Campus Virchow-Klinikum, University Medicine Berlin, Augustenburger Platz 1, 13353 Berlin, Germany
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Abstract
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The aim of this study is to evaluate the radiation dose to the interventional radiologist in transjugular intrahepatic portosystemic shunt (TIPS) concerning the risk of cancer and deterministic radiation effects and the relation to recommended dose limits. In 18 TIPS interventions radiation doses were measured with thermoluminescence dosemeters (TLD) fixed at the eyebrow, thyroid and hand of the radiologist without special lead shielding of these body parts and at the chest, abdomen and testes under the lead apron. The doses of the eye lens, thyroid gland and hand were assumed to be equal to the corresponding surface doses. The dose at the abdomen under the lead apron was used as an estimation of the ovarian dose. Effective dose equivalent was estimated by Webster's method. The estimated effective dose equivalent was 0.087 mSv and the effective dose 0.110 mSv. The risk of fatal cancer was of 10–6 and the risk of severe genetic defect of 10–7 for one single intervention. The maximum permissible number of TIPS interventions was 181, otherwise the dose limit for effective dose would be exceeded. When the radiologist performed more than 372 TIPS procedures per year for many years, the dose to the lens of the eye could exceed the threshold for cataract. If the interventionist performs a large number of TIPS procedures in a year, the risk of fatal cancer and developing cataracts becomes relatively high.
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Introduction
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A major source of radiation doses to medical personnel are fluoroscopic procedures [1]. Interventional angiographic treatments are typically associated with higher radiation exposure than diagnostic angiography. The transjugular intrahepatic portosystemic shunt (TIPS) has been shown as effective in the treatment of complications of portal hypertension such as variceal bleeding and refractory ascites [2, 3]. Furthermore, it was shown that TIPS is more effective in preventing variceal re-bleeding than the competitive endoscopic method [2]. Nevertheless, TIPS has been known as an intervention with the highest radiation exposure to patients undergoing abdominal angiographic procedures [4, 5].
In association with the high radiation exposure to the patient undergoing TIPS, it can be assumed that the interventional radiologist is exposed to high levels of scattered radiation. The International Commission on Radiological Protection [6] and the Food and Drug Administration (FDA) [7] has estimated the radiation risk of cancer and deterministic effects, such as cataract of the eye lens, and recommended dose limits.
The aim of this study is to evaluate the radiation dose to the interventional radiologist in TIPS concerning the risk of cancer, deterministic radiation effects and the relation to the recommended dose limits.
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Methods
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Radiation doses were measured in 18 TIPS procedures (18 patients, age 54.8±10.0 years; 15 male and 3 female) with liver cirrhosis and refractory ascites as complications of the portal hypertension. The intervention was performed with angiographic equipment (Multidiagnost; Philips, Eindhoven, The Netherlands) with under table tube and with pulsed fluoroscopy mode with 12 images per second.
Thermoluminescence dosemeters (TLD; Harshaw, Cleveland, OH) with lithium fluoride crystals (3.2 mmx3.2 mmx0.9 mm) doped with Mg (LiF:Mg) were used for dosimetry. The minimum detectable level was 0.01 mSv. As the total time between regeneration, exposure and next regeneration of the detectors was very short (3 days maximum), no correction for natural radiation (0.05 mSv month–1) had to be made. The fading rate of 5% in 12 months was also negligible. Taking into account the individual responsivity of the detectors to the energy of X-ray radiation and the filter that was used (2.6 mm Al), the energy correction factor was found to range from 1.00 for a tube voltage of 60 kV to 1.05 at 100 kV. Reproducibility was found to be 1–2% in doses more than 10 times greater than the detection limit.
To measure the radiation exposure to the radiologist (NH), one TLD was attached on both sides of the eyebrows and the thyroid gland over the lead apron, on the back of the hand at the base of the middle finger, on the chest at the breastbone, the abdomen at the navel and at the testes under the lead apron. The radiologist used a 0.35 mm lead equivalent lead apron. A lead glass with 1.0 mm lead equivalent was placed permanently in front of the patient chest between the image intensifier and the radiologist. Thyroid collar and lead glasses were not worn.
To establish a portosystemic shunt, a puncture needle was advanced transjugularly in a catheter through the inferior vena cava into the right hepatic vein. An intrahepatic branch of the portal vein was punctured and self-expandable bare nitinol stents (Angiomed, Karlsruhe, Germany) with a diameter of 10 mm and a length of 4 cm or 6 cm were implanted. The end of the stent was located within the hepatic vein. The stents and the hepatic vein were dilated to 10 mm after deployment. Pressure measurements were performed for the portal vein and right atrium in order to ensure that the portosystemic pressure gradient was reduced to 10–15 mm Hg. When TIPS placement was completed, the guide wire and angiographic sheath were removed.
Doses of the eye lens and thyroid gland were assumed to be equal to the corresponding measured surface doses. The dose at the abdomen under the apron was used as an estimation of the dose to the ovaries. The effective dose equivalent was estimated by Webster's method [8]. Webster developed an empirical method for estimating effective dose equivalent according to the ICRP 26 [9]. This method is based on organ dose measurements and effective dose calculations of Faulkner and Harrison [10] and uses the 1977 ICRP tissue weighting factors [9] to the under apron dose (Hu) and the over collar dose (Ho) with the equation effective dose equivalent = 1.5xHu+0.04xHo.
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Results
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Fluoroscopy time was 77.8±66.3 min, the dose–area product (DAP) 446.0±279.7 Gy cm2 (mean±standard deviation). The surface doses are shown in Table 1
. One surface dose at the thorax and under the apron was at the minimum detectable level. All other surface doses were above this level.
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Table 1. Radiation(surface) doses of the interventional radiologist as mean±standard deviation. (In brackets dosemeter position)
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The doses at the eyebrows and the thyroid gland were higher on the right side than on the left side. The left hand was exposed more strongly than the right hand and showed the highest dose among all body parts monitored by TLD.
Using Webster's method [8], the effective dose equivalent [9] was calculated by using the values of the over collar and under apron dosemeters. For radiologists who do not wear a thyroid collar, the effective dose equivalent calculated with Webster's method is 21% below the effective dose [6] calculated with organ dose tables and depth dose charts [11]. The effective dose would be 0.110 mSv and the maximum permissible number of TIPS would be 181 per year (Table 2).
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Table 2. Comparison of the doses of the interventional radiologist with the recommended dose limits and the maximum permissible numer of TIPS interventions per year
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To meet the recommended dose limit [6], the maximum permissible number of TIPS interventions was estimated to be 181, otherwise the dose limit for the effective dose would be exceeded (Table 2
). A pregnant radiologist could perform a maximum of 23 interventions after the declaration of pregnancy during the remainder of the pregnancy (Table 3). The risk of fatal cancer was approximately 10–6 and the risk of severe genetic defect 10–7 for a single intervention (Table 4). If a radiologist performed more than 248 TIPS per year for many years, the radiologist could reach the threshold for radiation-induced detectable opacity of his eye lens. For a workload of more than 372 times a year and for many years, he could possibly develop a cataract (Table 5).
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Table 3. Comparison of the dose to the abdomen of the interventional radiologist with the recommended dose limit for the conceptus of a pregnant woman and the maximum permissible number of TIPS interventions after the declaration of pregnancy during the remainder of the pregnancy
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Table 5. Comparison between the measured doses of the radiologist and threshold for radiation-induced deterministic damage [6, 7]
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Discussion
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In angiography, the patient is exposed by direct radiation and the radiologist by scatter radiation. However, the radiologist may work with and be exposed to radiation every day for many years. In a previous, study with a very limited number of patients undergoing transarterial hepatic chemoembolisation, we showed that the dose to the eye lens is a quantity that may limit the permissible number of interventions before a dose limit would be exceeded [12]. This study shows that it may be deduced from one to two TIPS interventions per day, the dose limit for the eye lens can be exceeded. It also reveals that the effective dose can be a limiting quantity for the number of the interventions, as one TIPS per day can cause the cumulative effective dose to be above the limit after 1 year. In the case of pregnancy, one TIPS per day can cause the dose limit recommended by the ICRP for the unborn child be exceeded after 24 days.
The risks of fatal cancer derived from effective dose and of severe genetic defect are extremely small for the radiologist after one single TIPS. However, for high workloads, e.g. 500 per year and over 10 years, these stochastic risks can increase to a magnitude of 10–3 to 10–2.
The radiologist should be aware of the radiation exposure to the eye lens and the associated possibility of cataract when he performs interventions like TIPS one to two times per day over many years. Sterility will not occur as a deterministic radiation effect to personnel after TIPS.
The dose differences between the right and left side of the eye lens, thyroid gland and hands can be explained by the position of the radiologist in relation to the X-ray tube. While standing at the head of the patient during the TIPS intervention, the tube is mostly located at the liver side, i.e. right side of the patient. Thus, the distance of the right lens and side, of the thyroid gland to the tube is shorter than on the left side. The left hand was used to guide and rotate the catheter and placed at the neck, while the right hand was placed at the proximal end of the catheter to inject contrast media into the catheter. Thus, the left hand was closer to the tube than the right hand. Therefore, in the interests of the radiologist, dose measurements in interventions should be performed at both sides when it is not clear which side is irradiated more strongly.
Not only can there be dose differences between the sides, but the dose distribution at the same hand may not be uniform. Whitby and Martin [13] have found that for most interventional radiological procedures the bases of the ring and little finger receive the highest dose. In this study we measured the dose at the base of the middle finger, so that the maximum dose to the hand could be higher than we have found.
Organs or body parts located near the surface, such as the eye lens, may be measured on the surface. For the estimation of the ovarian dose we used the surface dose to the abdomen. In this case, it can be argued that the lead apron shields the individual from radiation, preferentially at lower energies. The parts of the energy spectrum which penetrates the lead apron are the higher energies and these should not be absorbed significantly by the abdomen on their way to the ovaries. Thus, the surface dose under the apron should be a reasonable estimation of the ovarian dose. We did not measure the dose to the leg. A study by Whitby and Martin [14] showed that in TIPS, the leg dose could be as much as two to three times greater than that to the hands. In contrast, Meier et al [5] have found that the dose to the hand was the highest in TIPS (about 0.7 mSv at the hand and 0.2 mSv at the leg).
Regarding radiation dose to the radiologist, there are two particular aspects of TIPS. First, it is associated with a long fluoroscopy time [4]. This is because the catheterization of the hepatic vein, puncture and catheterization of the portal vein must be guided by fluoroscopy and is often difficult. Normally, TIPS is performed by an experienced interventionist. Second, many patients needing a TIPS have large amount of ascites, which automatically leads to higher voltage and tube current. In big departments with many different radiological procedures, the competences of the physicians are divided because no one can be really specialized in all of these procedures. Those who are competent for TIPS and perform about 100 of these interventions per year accumulate more radiation dose and may have less reserve for different interventions than others who, for example, perform similar numbers of angioplasties of the lower extremity.
To protect personnel from stochastic and deterministic risks, measures of radiation protection should be used when interventions such as TIPS are performed frequently. The distance to the X-ray source should be kept as great as possible as the dose is reduced with the square of the distance. Lead apron, thyroid collar and lead glasses may greatly reduce the operator radiation exposure in cardiac interventions (to 0.8%) [15]. Thyroid collars could reduce the effective dose by approximately a factor of two [11]. Permanent lead shielding, e.g. a lead plate as we have used, can be placed between the patient and the radiologist to reduce the amount of scatter radiation reaching the trunk and head of the radiologist. Lead gloves have been found to be flexible and could lead to dose reduction of about 20% [16]. For a number of years we have used the pulsed fluoroscopy mode with 12 images per second and cannot see significant reduction of the image quality during the intervention. Zweers et al [17] described significant reduction of the estimated staff effective dose in TIPS using dedicated fluoroscopy exposure settings. Last but not least, a recently described MR guided TIPS with use of a hybrid radiography/MR system should facilitate the puncture of the portal vein and be associated with less radiation exposure than a conventional TIPS procedure [18].
Received for publication March 14, 2005.
Revision received September 7, 2005.
Accepted for publication September 19, 2005.
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References
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Abstract
Introduction
