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First published online April 26, 2006
British Journal of Radiology (2006) 79, 644-651
© 2006 British Institute of Radiology
doi: 10.1259/bjr/76128583
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Occupational exposure in the electrophysiology laboratory: quantifying and minimizing radiation burden

N Theocharopoulos, MSc1,2, J Damilakis, PhD1, K Perisinakis, PhD1, E Manios, MD3, P Vardas, MD, PhD, FESC, FACC3 and N Gourtsoyiannis, MD, PhD4

1 Departments of Medical Physics, 3 Cardiology and 4 Radiology, Faculty of Medicine, University of Crete, P.O. Box 2208, Iraklion 71003, Crete and 2 Department of Natural Sciences, Technological Education Institute of Crete, P.O. Box 140, Iraklion 71004, Crete, Greece

Correspondence: J Damilakis, Assistant Professor, Department of Medical Physics, Faculty of Medicine, University of Crete, P.O. Box 2208, Iraklion 71003, Crete, Greece


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 
Fluoroscopically guided procedures in the electrophysiology room, such as radiofrequency catheter ablation and implantation of cardiac resynchronization devices, may result in high radiation exposure of electrophysiologists and assisting staff. Our aim was to provide accurate and applicable data on occupational doses to the electrophysiology laboratory personnel. We exposed fluoroscopically an anthropomorphic phantom at three projections common in electrophysiology studies. For each exposure, scattered radiation was measured at 182 sites of the cardiology room at four body levels. Effective dose values, eye lens, skin and gonadal doses to the laboratory staff were calculated. Our study has shown that a procedure requiring 40 min of fluoroscopy yields a maximum effective dose of 129 µSv and a maximum value of gonadal dose of 56.8 µSv to staff using a 0.35 mm lead-equivalent apron. A conservative estimate of the electrophysiologist's annual maximum permissible workload is 155 procedures. Staff effective dose values vary by a factor of 40 due to positioning during fluoroscopy and by a factor of 11 due to radiation protection equipment. Undercouch protective shields may reduce gonadal doses up to 98% and effective dose up to 25%. Consequently, radiation levels in the electrophysiology room are not negligible. Mitigation of occupational exposure is feasible through good fluoroscopy and working practices.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 Conclusions
 References
 
In recent years, due to technological advances, a plethora of new fluoroscopically guided procedures has been introduced in the electrophysiology suite [1–9]. Radiofrequency catheter ablation and implantation of rhythm devices and cardioverter defibrillators have been proven to supersede the effectiveness of other therapeutic approaches and, as a result, their application is increasing [7, 8]. Hence, electrophysiologists and support personnel may be exposed to considerable levels of radiation, depending on the laboratory workload and complexity of the procedures [9–11]. The need for quantifying the risk of radiation detriment to laboratory staff is imperative.

Because of the different radiosensitivities of the various body organs and tissues, for non-homogeneous irradiations, radiation detriment is assessed by a radiation quantity termed effective dose, introduced by the International Commission on Radiological Protection (ICRP). The effective dose E is the weighted average of the mean absorbed dose DT to 22 organs/tissues, where the organ/tissue-specific weighting factor wT is the fractional organ/tissue contribution to the total body detriment [12]:



Formula 001

The risk of genetic effects, i.e. impairment on progeny, depends on the preconceptual radiation dose delivered to staff gonads. Other organ/tissue doses of interest are the skin and the eye lens dose, which together with effective dose, are controlled by regulatory annual limits [12].

To the best of our knowledge, there is no reported experience on occupational effective and gonadal doses as well as maximum permissible workloads regarding the electrophysiology suite. The present study aims to provide accurate and applicable data on occupational doses to the electrophysiology laboratory personnel.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 Conclusions
 References
 
Simulated exposures and scatter measurements
An anthropomorphic tissue-equivalent phantom (RANDO; Alderson Research Labs, Stanford, CA) simulating an adult was positioned supine on the surgical table and fluoroscopically exposed at three distinct projections common in interventional cardiology: (a) posteroanterior (PA), (b) left anterior oblique 45° (LAO) and (c) right anterior oblique 25° (RAO). A mobile undercouch C-arm fluoroscopic unit was used (Philips BV300-R2; Philips Medical Systems, Best, The Netherlands) at a focus to image intensifier distance of 100 cm. The half value layer of the X-ray tube was 4.7 mm aluminium at 70 kVp and the diameter of the input field size was 23 cm.

Scatter air kerma rates were measured using a hand-held ionization chamber (LB 1236, EG&G BERTTHOLD ) connected to a portable digital meter (UMo LB 123, EG&G BERTTHOLD). The operating theatre floor was divided into two grids relative to the long sides of the table. The grids were 3 mx1.5 m in dimensions and consisted of 25 cmx25 cm square cells. 91 measuring points were thus defined on each grid. Measurements were performed at the height of the gonads (80 cm above the floor), the waist (100 cm), the neck (150 cm) and the face (165 cm). The air kerma readings were divided by the dose–area product (DAP) rate of each exposure to obtain data independent of exposure parameters. All measurements were repeated with two removable flexible undercouch 0.5 mm lead-equivalent protective shields (60 cmx70 cm) attached to the table's long sides (Mavig, Muenchen, Germany).

Calculation of occupational doses
Using coefficients provided by ICRP [13], the face level air kerma measurements were converted into eye lens dose and to personal dose equivalent Hp (0.07)F [14]. The latter corresponds to the dose received by the facial skin. The neck level air kerma measurements were converted into personal dose equivalent Hp (0.07)N while the waist level measurements were converted to personal dose equivalent Hp (10)W. The genital levels measurements were converted to male and female staff gonadal dose with use of coefficients provided by ICRP [13]. Previously published attenuation data were used for the calculation of gonadal and deep doses under a 0.35 mm and 0.50 mm lead-equivalent apron, and of the eye lens doses under 0.35 mm lead-equivalent goggles [15]. The polychromatic nature of X-rays was accounted for by weighting the available monoenergetic dose conversion coefficients on the basis of the characteristics of the utilized spectra.

The values of Hp W under apron protection and the Hs N values at neck level were used for the calculation of staff effective dose according to the following equations [16]:



Formula 002

when a thyroid shield is not used, and


Formula 003

when a thyroid shield is used.

Equation (1) was also used for the calculation of effective dose to unshielded personnel by means of attenuation-free Hp (10)W values.

The neck level air kerma measurements were also converted into to personal dose equivalent Hp (10)N, to enable the correlation of the reading of an over-apron chest dosemeter with the actual value of effective dose to monitored staff. To investigate the effect of beam quality on staff doses, the Hp (0.07)N, Hp (10)W and Hp (10)N values were calculated for four tube voltage settings: 70 kVp, 85 kVp, 100 kVp and 120 kVp.

Calculation of radiation burden from a complex procedure
Derived occupational doses are projection specific and DAP normalized since they originate from the initial DAP normalized scatter radiation measurements. This normalization eliminates the dependence on exposure technique factors (kilovoltage and milliamperage) or instrumentation and enables occupational dose calculations from any complex procedure on the basis of the individual DAP value of each projection involved [17].

The fluoroscopy time required per patient strongly depends on the type of electrophysiological study. Reported fluoroscopy course durations range from 3.54 min per procedure for conventional cardiac rhythm device implantation [18] to 77 min per patient for biventricular pacing [10]. For the estimation of the staff radiation burden per patient, a complex procedure requiring 40 min of fluoroscopy was considered. The total DAP of the examination was assumed to be 4300 cGy cm2 and the relative contribution of the PA, RAO and LAO projection was 58%, 15% and 27%, respectively [3]. Adopted fluoroscopy time and total DAP value are similar to those derived from patient studies performed at our institution regarding biventricular pacing (35.2 min and 4765 cGy cm2) [19] and radiofrequency catheter ablation (41 min) [3].

For the hypothesised complex procedure, staff doses and workloads were calculated at three positions at the left hand side of the patient and at the symmetrical positions at the right hand side of the patient. Two sites of interest adjacent to (12 cm from) the surgical table were considered, at the level of the patient's groin (femoral area) and heart (subclavian position), while the third position was selected 1 m away from the operating table, also at the level of the patient's heart.

The effect of projection ratios constituting a complex procedure on staff effective dose values was investigated by considering an alternative procedure in which the roles between the LAO and RAO projections were interchanged, i.e. the RAO contribution was increased by a factor of 1.8 and the LAO contribution was decreased by a factor of 0.56.

Calculation of workloads
Maximum annual permissible workloads were calculated on the basis of the ICRP recommendations regarding effective dose (20 mSv per year), eye lens dose (150 mSv per year) and skin dose (500 mSv per year) [12].


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 Conclusions
 References
 
Distribution of effective dose values per projection
The highest occupational exposure during a PA or RAO projection is delivered next to the patient's chest at the left hand side (subclavian position), while during a LAO projection the symmetrical site at the right hand side of the patient receives the highest radiation burden (Figure 1Go). The maximum values of effective dose to the electrophysiology laboratory staff, protected by 0.5 mm lead-equivalent apron and thyroid collar, are 14 nSv cGy–1, 14.8 nSv cGy–1 and 10.5 nSv cGy–1 for the PA, RAO and LAO projection, respectively.


Figure 1
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  		Figure 1. Distribution of effective dose in the electrophysiology laboratory to 0.5 mm lead-equivalent apron and collar protected staff from (a) posteroanterior (PA), (b) right anterior oblique 25° (RAO) and (c) left anterior oblique 45° (LAO) heart fluoroscopy. Provided values are dose–area product (DAP)-normalized and expressed in nSv cGy cm–2.

 
Radiation burden from complex procedure
During an electrophysiological study, the highest radiation levels occur adjacent to the patient's heart. A complex 40 min procedure yields between 54.2 µSv and 129 µSv depending on radiation protection measures (Table 1Go). Effective dose values are considerably lower at the right hand side of the patient where, at 1 m from the table, they range from 3.38 µSv to 8.47 µSv, depending on protective equipment used. The annual permissible workload derived for the effective dose constraint is at minimum for a staff member using only a 0.35 mm lead-equivalent apron while occupying the site of highest exposure, and equals 155 procedures. Since cancer risk increases with effective dose and workloads decrease with dose, their variation with staff positioning and protection devices can be readily appreciated.


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  		Table 1. Effective dose and maximum permissible annual workloads for electrophysiology laboratory staff, calculated for four radiation protection conditions: (a) 0.35 mm lead-equivalent apron, (b) 0.35 mm lead-equivalent apron and 0.5 mm lead-equivalent collar, (c) 0.5 mm lead-equivalent apron and (d) 0.5 mm lead-equivalent apron and 0.5 mm lead-equivalent collar

 
Interchanging roles between the LAO and RAO projections, as in the alternative complex procedure considered, would increase effective doses to staff working adjacent to the tableside at the left hand side of the patient by 10.1%, and reduce effective dose values at the right hand side of the patient by 10.6%.

By analogy to effective dose, eye lens and face skin dose present their peak values, 389 µSv and 446 µSv per procedure, respectively, at the left hand side of the patient (Table 2Go). Maximum permissible workloads derived from the skin dose limit always exceed that derived from the eye lens dose constraint, unless eye protection is used.


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  		Table 2. Eye lens dose, face skin dose and corresponding maximum permissible annual workloads for the electrophysiology laboratory staff

 
Gonadal doses are also higher at the left hand side of the patient. Moreover, male gonads receive approximately twice the dose delivered to female gonads. The maximum estimated gonadal dose was 56.8 µSv per procedure for the male staff using a 0.35 mm lead-equivalent apron (Table 3Go).


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  		Table 3. Female and male gonadal doses for the electrophysiology laboratory staff protected by(a) 0.35 mm lead-equivalent apron and (b) 0.5 mm lead-equivalent apron

 
Correlation between effective dose and dosemeter reading
An over-apron dosemeter worn at chest height records an Hp value that is from 3.6 to 27 times higher than the effective dose actually received by the monitored staff member in the electrophysiology laboratory, depending on radiation protection equipment used and the tube voltage (Table 4Go).


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  		Table 4. Ratios of dosemeter reading* to effective dose, for an over-apron chest dosemeter

 
Effective dose and tube voltage
The penetrating ability of radiation through protective apron and tissue increases with beam energy, and hence effective dose values depend also on tube voltage. The correction factors of Table 5Go show the effect of voltage on staff effective dose and can be used as multipliers to adjust the data of Table 1Go or Figure 1Go, which were derived for 100 kVp.


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  		Table 5. Correction factors(multipliers) for effective dose calculation relative to data of Table 1Go and Figure 1Go

 
The effect of removable protective shields
For the complex 3-projection procedure considered, the effect of the removable under-apron protective drapes on gonadal doses is markedly different from that on effective dose values, within the confines of the electrophysiology laboratory (Figure 2Go). Effective dose is reduced up to 20% for staff members working at the left hand side of the patient while for those working at the contralateral side, reduction up to 40% occurs in the vicinity of the patient's groin and up to 50% at 0.5 m from the couchside. The reduction in gonadal doses is also asymmetrical. However, protection is more effective since staff gonadal doses are reduced by 90–98% at both sides of the patient's groin. It is evident that the undercouch shields protect mostly the lower part of the body.


Figure 2
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  		Figure 2. Percent reduction of(a) effective dose and (b) gonadal dose delivered to electrophysiology laboratory staff from the complex three-projection procedure considered, accomplished with use of removable undercouch protective drapes.

 

   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
Conclusions
 References
 
Applicability of provided dosimetric data
A recent study has shown that scattered radiation patterns from fluoroscopically exposed patients are similar to those obtained from simulated exposures of a RANDO phantom and that variations in the body habitus have a minor impact on scatter air kerma rates [20]. Hence, presented data allow the accurate estimation of staff occupational doses and radiogenic risks in the electrophysiology laboratory. Although not explicit, doses of GoGoTables 1–3Go are in effect DAP-normalized, since a strictly defined complex procedure of 4300 cGy cm2 was considered. Hence, staff doses in any institution from procedures similar in geometry to the procedure adopted in the current study may be estimated on the basis of the tabulated data, corrected for total DAP value. Even major alterations in the ratios of the constituent components of fluoroscopic imaging will have minor impact on doses to tableside workers. However, if more accurate calculations are required, the isodose maps of Figure 1Go and the individual DAP values of the constituent projections may be used. Effective dose estimation using the above methods is demonstrated in the following example. A cardiologist has performed an electrophysiology study positioned next to the patient's right hip (femoral area, x = –0.5, y = 0). The total DAP value recorded during fluoroscopy at 85 kVp was 5000 cGy cm2 and the estimated relative contribution of PA, LAO and RAO projections is 60%, 25% and 15%, respectively. From the data of Table 1Go and the high voltage correction factors of Table 5Go, the effective dose (E) received by the cardiologist may be obtained as follows (1st method):



Formula 004

More precisely (2nd method), the isodose curves of Figure 1Go can be combined with Table 5Go correction factors to yield E:



Formula 005

A third, retrospective, method proposed for effective dose estimation is based on personal dosemeter reading and the conversion coefficients of Table 4Go. Suppose that during an electrophysiological study performed at an average voltage of 70 kVp, the over-apron dosemeter positioned over a 0.35 mm lead-equivalent apron records an Hp N value of 4.7 mSv. The actual effective dose delivered will be:



Formula 006

The actual radiation risks
Occupational exposure in the electrophysiology laboratory is most unlikely to incur deterministic effects to staff. Not only are dose limits well below the threshold of the induction of such effects, but also the fluoroscopy times required for the accumulation of threshold doses are extremely high. The 2 Sv-threshold for the induction of erythema [12] will be reached after at least 3000 h of fluoroscopy, while cataract formation occurs after at least 1700 h of fluoroscopy or 1 Sv. The threshold dose for temporary sterility under prolonged exposure is 0.4 Gy per year to male gonads [12], which corresponds to 2240 h of fluoroscopy. Hence, the actual concern for the laboratory staff is the induction of stochastic effects, such as cancer and impairment to progeny.

Factors affecting occupational doses
The quantity directly related to the risk of cancer induction and used for the expression of occupational dose limits is effective dose. Genetic risk, on the other hand, calls for the estimation of gonadal doses to staff. Our study has shown that, within a 1.5 m radius from the tableside, effective dose varies by a factor of 40 due to staff positioning and by a factor of 11 due to differences in radiation protection measures. Gonadal doses present even higher discrepancies. Hence, providing limited data regarding scatter dose levels, or even effective doses, without exact information on beam geometry, location or protective devices, is of little value in developing a radiation protection strategy. Mapping the electrophysiology laboratory in terms of effective dose and gonadal dose for several beam geometries and different protective equipment is essential for the precise calculation of staff radiogenic risk. Moreover, the use of any available removable radiation barriers should be accurately duplicated in a separate set of measurements, since there is no other reliable method of predicting their efficiency in dose reduction [21, 22].

Maximum permissible workloads
For each working site of interest, the overall maximum permissible workload is the minimum of the three partial workloads imposed by the constraints on eye lens, face skin and effective dose. Since workloads depend on positioning and protection measures, a simplified and safe approach is to adopt the workload derived for the most burdened position with the minimum protection of a 35 mm lead-equivalent apron. Hence, no dose limit will be violated if the maximum DAP value of fluoroscopy performed annually is less than 0.7 Gy m2. This corresponds to 6200 min of fluoroscopy per year or 155 procedures.

Comparison with previous studies
Calkins et al [24] measured radiation exposure to medical personnel during radiofrequency catheter ablation using thermoluminescent dosimetry. Using the mean exposure, they recorded per case at waist and thyroid level (532 µSv and 156 µSv, respectively) and by assuming a 0.5 lead-equivalent apron and collar, the current study methodology yields a mean effective dose value of 15.2 µSv for 44 min of fluoroscopy. Lindsay et al [25] estimated an effective dose equivalent, which is the predecessor of effective dose, of 18 µSv per case or 55 min of fluoroscopy when a thyroid shield is used, and of 28 µSv per case without thyroid protection, for the physician located at the femoral area. The effective dose equivalent was approximately twice as high near the subclavian position. Regarding eye lens dose, Calcins et al [24] reported 281 µSv per case, Vano et al [26] 294 µSv and Kuon et al [21] more than 165 mSv per year for an annual workload of 1000 interventions. Given the multiplicity and complexity of factors affecting occupational doses, reported data are in broad agreement with the current study estimates. It should be stressed, however, that the vast majority of previously reported occupational doses are based on fluoroscopy duration observations and not on DAP. Hence, they are strongly dependent on fluoroscopy technique factors and equipment used, and cannot serve the purpose of accurate dose estimation in other electrophysiology laboratories.

Mitigation of radiation risks
Two easy-to-apply practices are recommended for minimizing personnel exposure in the electrophysiology laboratory. Approaching the patient from the right hand side, specifically at the groin level, is preferable since the radiation backscatter effect is less pronounced. Also, reducing tube voltage and milliamperage is beneficial for staff doses in two ways: the total DAP per patient is diminished and staff effective dose values per unit DAP fall as the beam energy decreases. Personal protection is also extremely important. A 0.35 mm lead-equivalent apron reduces effective dose to staff by a factor of 10, compared with the unshielded values. Additional use of a 0.5 mm thyroid collar provides a further 1.5-fold decrease. Use of both 0.5 mm lead-equivalent apron and collar provides a protection factor of 26. Furthermore, operators should avoid or limit the use of LAO projection when positioned on the right side of the patient and of RAO projection when positioned on the left [22]. Other good fluoroscopy practices, such as strict beam collimation use of pulsed fluoroscopy and last frame hold feature, act to reduce radiation administered per patient and keep staff radiation doses as low as reasonably achievable [21, 27, 28]. However, the degree of radiation reduction depends on specific characteristics of the equipment used. Therefore, optimization of imaging protocols should be based on a comprehensive evaluation of the dosimetric characteristics and the performance of automatic exposure control of the employed fluoroscopic unit, preferably in terms of DAP rate [29].

Limitations of the study
The major limitations of the present study are that not all of the beam geometries possibly encountered in electrophysiological studies were simulated, and that no ceiling-suspended radiation barriers were available in our institution. However, the selected projections are the major constituents of any cardiac study. Moreover, the effect of commercially available removable radiation barriers should be individually examined at each institution, according to their specific use, with extensive radiation measurements regarding beam angulation and staff/barrier positioning. Furthermore, since the distribution and attenuation of scatter is likely to be influenced by the volume of tissue lateral to the scattering site, real patients, who may differ considerably in size compared with the RANDO phantom, may produce a somewhat different scatter pattern.


   Conclusions
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
References
 
Radiation hazards in the electrophysiology laboratory should not be overstated nor ignored. Continuous risk assessment and minimization is required. Good fluoroscopy and radiation protection practices to mitigate occupational exposure to radiation should be proposed and implemented.

Received for publication June 7, 2005. Revision received December 9, 2005. Accepted for publication January 3, 2006.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 

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