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Occupational radiation doses in interventional cardiology: a 15-year follow-up

¹ Department of Radiology, Complutense University Medical School 28040 Madrid, Spain, ² San Carlos University Hospital, Medical Physics Service, ³ Cardiovascular Institute, 28040 Madrid, Spain

Correspondence: Prof. Luciano Gonzalez

	Abstract

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
This report describes occupational radiation doses of interventional cardiologists over 15 years and assesses action undertaken to optimize radiation protection. Personal dosimetry records of nine staff cardiologists and eight interventional cardiology fellows were recorded using personal dosemeters worn over and under their lead aprons. The hospital in which this study was conducted currently performs 5000 cardiology procedures per year. The hospital has improved its facilities since 1989, when it had two old-fashioned theatres, to include four rooms with more advanced and safer equipment. Intensive radiation protection training was also implemented since 1989. Initially, some individual dose values in the range of 100–300 mSv month^–1, which risked exceeding some regulatory dose limits, were measured over the lead apron. Several doses in the range of 5–11 mSv month^–1 were recorded under the apron (mean = 10.2 mSv year^–1). During the last 5 years of the study, after the implementation of the radiation protection actions and a programme of patient-dose optimization, the mean dose under the apron was reduced to 1.2 mSv year^–1. Current mean occupational doses recorded under the lead apron are 14% of those recorded during 1989–1992 and those recorded over the apron are 14-fold less than those recorded during 1989–1992. The regulatory dose limits and the threshold for lens injuries might have been exceeded if radiation protection facilities had not been used systematically. The most effective actions involved in reducing the radiation risk were training in radiation protection, a programme of patient-dose reduction and the systematic use of radiation protection facilities, specifically ceiling-suspended protective screens.

	Introduction

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Radiation exposure is a significant concern for interventional cardiologists (ICs) because workloads and the complexity of procedures have increased over the past few years without a corresponding increase in the number of specialists [1]. Although reduced scatter radiation in catheterization laboratories compared with that in old X-ray system laboratories, improved radiological protection facilities, and better, more inclusive radiation protection training for ICs have substantially reduced the risk of radiation exposure, the complexity and number of procedures have increased. Therefore, interventional cardiology is recognized as a high-radiation-risk practice [1–3], and evaluation and follow-up of occupational doses should be considered an important part of quality assurance (QA) programmes.

Several aspects of radiation safety in the practice of cardiology have been addressed by the American College of Cardiology in a consensus document [4]. The UNSCEAR 2000 report [5] states that fluoroscopic procedures are by far the largest source of occupational exposure in medicine. Cardiac catheterization, in particular, can represent a major source of exposure. A study performed in the UK [6] indicated that ICs receive a mean annual dose of 0.4 mSv, twice that received by radiologists and many times that received by nurses and technicians.

There are substantial differences in occupational doses between cardiac laboratories [7–10]. This is caused by differences in X-ray systems (old film-based systems versus digital units) and their particular settings, levels of training in radiation protection, frequency of use of radiation protection facilities and personal dosemeters, and workloads of specialists.

Renaud et al [11] described a 5-year follow-up of the radiation doses received by the in-room personnel of three cardiac catheterization laboratories and concluded that some workers may have exceeded the occupational limit for the lens of the eye. Lens injuries have been reported for several interventional radiology suites in which radiation protection conditions were not appropriate to the level of risk [12].

This report describes occupational radiation doses from interventional cardiology in a university hospital over a period of 15 years and the actions that were taken to optimize radiation protection. Data were gathered from a dosemeter worn on the trunk of the body under the apron and a dosemeter worn outside the apron, as recommended by the International Commission on Radiological Protection (ICRP) [1].

	Methods and materials

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Follow-up of IC's personal dosimetry records was performed in a university hospital currently performing more than 5000 procedures per year in four catheterization laboratories with nine staff cardiologists and eight fellows. In 1989, this interventional cardiology service used two old-fashioned X-ray units. In 1994, a Philips Optimus M-200 Poly C X-ray unit (Philips, Best, The Netherlands), installed in 1988, was upgraded and an old CGR unit was exchanged for a Philips Integris HM-3000. In 2000, two new Philips Integris H-5000 units were installed. All systems now have protective screens suspended from the ceiling. This radiation protection tool, which had previously not been installed in one of the rooms, was not used regularly by all specialists until they were made aware of its importance. In addition, lead aprons, thyroid protectors and lead glasses were also available and are used routinely at present (with a few exceptions).

Two personal dosemeters with thermoluminescent dosimetry chips, as recommended by the radiation protection service of the hospital, were used for occupational dosimetry: one was worn on the trunk of the body under the apron and the other was worn outside the apron at the level of the collar or the left shoulder. A dosemeter under the apron provides an estimate of the dose to the organs of the shielded region. A dosemeter worn outside the apron supplies an estimate of the dose to the organs of the head and neck, including the thyroid and lenses of the eyes (if unshielded), but greatly overestimates the doses to organs of the trunk. Results obtained from both dosemeters were used to estimate the occupational effective dose as recommended by the NCRP [13] and ICRP [1]. Dosemeters were read monthly by a public dosimetry service accredited and audited by the National Regulatory Authority.

Before 1992, training in radiation protection for ICs was scant, if performed at all. Subsequently, a radiation protection training programme was initiated in accordance with national regulations [14]. Of the staff cardiologists working in the centre, 90% attended the courses and were accredited in radiation protection, as required by the National Regulatory Authority. Some new cardiologists, especially fellows, did not attend the courses. New regulations in force since 1999 [15] require a second level of radiation protection training for interventionalists, which includes training in radiation protection of patients and QA, as recommended by the ICRP [1]. Training in radiation protection of patients is also required by European Directive 43/97/EURATOM [16].

An interactive CD-ROM, co-sponsored by the European Commission [17], is used to provide radiation protection training for residents and fellows who commence work in interventional suites during the intervals between radiation-protection training courses. A copy of this CD-ROM is given to all new doctors on commencement of duty at the hospital's interventional cardiology service. In addition, refresher sessions on radiation protection are presented periodically.

Detailed analysis of personal dosimetry records of IC personnel is conducted every month. This is followed by individual interviews with persons exposed to monthly doses greater than 1.0 mSv under the apron (1/20 of the annual effective dose limit) or greater than 7.5 mSv over the apron (1/20 of the annual lens dose limit). In addition, a progressive audit programme was implemented to detect high patient doses, facilitate clinical follow-up in cases of likely skin radiation injury and to implement corrective action when necessary. Since 1999, a national standard [15] stipulates that patient doses in interventional procedures must be estimated and recorded. Because this patient-dose audit has reduced patient doses, occupational doses have also been reduced [2].

Personal dosimetry services typically provide monthly estimates of H_p(10) (the dose equivalent in soft tissue at 10 mm depth), which is usually compared with the annual limit of effective dose and with the eye lens limit [18], and H_p(0.07) (the dose equivalent in soft tissue at 0.07 mm depth) [18]. Usually, no significant differences between values are found in cardiac catheterization suites. The values reported in this paper are for estimates of H_p(10) obtained from personal dosimetry readings.

The effective dose, E, can be estimated [13] from the dosemeter values for H_w (under the apron at the waist, although this position is not critical) and H_n (above the apron at the neck) from the equation:

NCRP report 122 [13] contains specific recommendations for calculating the effective dose when protective aprons are worn during diagnostic and interventional medical procedures involving fluoroscopy. In addition to the above formula, it states that the effective dose can be estimated as H_n/21 if only one dosemeter is worn on the neck outside the apron.

	Results

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
The data from occupational dosimetry were allocated to one of three periods for purposes of analysis.

First period (1989–1992): investigation of high dose values and implementation of a customized radiation protection programme
Table 1 shows the findings from this period. Most values were in the range of 100–300 mSv month^–1, but in one case a dose of 1600 mSv month^–1 was recorded by the left shoulder dosemeter outside the lead apron. Values in the range of 5–11 mSv month^–1 were recorded under the apron. An initial complete evaluation of the radiation protection conditions of the catheterization laboratories was done, after which follow-up of abnormal values was investigated and corrective actions proposed. Consequently, the occupational medical service of the hospital advised some staff to abstain from catheterization duties for several months. The National Regulatory Authority was informed of these actions. Lens injuries would have occurred in those situations if the corrective actions had not been put into practice immediately.

Third period (1999–2004): implementation of occupational radiation protection in the QA programme
During this period, the frequency of the X-ray system QC programme increased from once yearly to two or three times per year. The old CGR X-ray system was removed in 1999. Full characterization was done by measuring patient entrance dose, image quality and scatter radiation levels for all fluoroscopy and cine modes. Closer contact with the maintenance engineers was established to customize the operation modes to fulfil the image quality requirements of the cardiologists while keeping doses as low as possible. Since this initiative, patient dose values were measured, recorded in a database and analysed periodically.

Since 2000, the MARTIR training CD-ROM [17] has been distributed to new personnel joining the interventional cardiology service, and radiation protection refresher seminars are held two or three times per year. Individual real-time occupational dosimetry has also been implemented for some procedures. Electronic dosemeters (Unfors EED-30; www.unfors.se) measure the dose accumulated by the specialist throughout a procedure and the maximum dose rate, which provides information about the correct use of the protective screen.

Maximum values recorded by dosemeters placed over the apron were lower during the third period than during the second period and ranged between 3 mSv month^–1 and 4 mSv month^–1. The maximum dose under the apron was generally 2 mSv month^–1, but some abnormally high values were recorded for specialists doing electrophysiology cardiac procedures (in service since 2000). A maximum over-the-apron dose of 26 mSv month^–1 was recorded for one specialist.

The workloads during the three periods were similar: five to six procedures per day and room, shared between one to three cardiologists. Some of the fellows stayed at the hospital for short periods and often performed many procedures per day to improve their skills. Typical workloads were two to four procedures per day for staff and three to six procedures per day for fellows. Table 2 shows monthly doses before, during and after radiation protection training. Mean and median doses decreased significantly after the training courses.

Table 4 presents estimates of the transmitted fraction of energy across different lead aprons with thickness equivalents in the range of 0.25–0.5 mm lead. The IPEM software application [19] for spectra from 70 kVp to 90 kVp was used for calculations.

In summary, the radiation protection programme during the 15-year period reduced the effective dose to cardiologists by one order of magnitude, avoiding cases of high individual doses. The real mean effective dose for cardiologists in our centre during the last 4 years of our study was 1.2 mSv year^–1, which is compatible with results recently reported by Delichas et al [20] (1.2–2.7 µSv/procedure, a dose of 0.7–1.5 mSv year^–1 for a workload of 50 procedures per month).

	Discussion

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Several questions arise from the results presented in this paper. First, it should be determined if the high doses reported during the years 1989–1992 are real dose values or incidental readings caused by inappropriate use of the dosemeters. In fact, the bulk of the results in Table 1 should correspond to real dose values received by the cardiologists during a period in which there was no culture of safety: ceiling-suspended screens were absent or unused, the X-ray systems were used in relatively high-dose fluoroscopy modes and film cine acquisition was done at 25 frames s^–1. The high dose values shown in Table 1 cannot be considered a consequence of the incorrect use of the dosemeters. All abnormal doses were reported to the doctors wearing the dosemeters and investigated with them, and no reason was found to suggest that incidental dosemeter irradiation occurred.

For the 1640 mSv measured at the left shoulder of a visiting cardiologist in 1 month, it was not possible to prove any abnormal dosemeter irradiation. The dose measured by the dosemeter worn under the apron was 42 mSv in that month, 2.8% of the dose over the apron. This figure is compatible with the transmitted fraction across the lead apron (Table 4). Moreover, experimental measurements in one of the cardiology rooms used by a fellow simulating clinical conditions produced doses in good agreement with the dosemeter readings, taking into account the presumed work rate, fluoroscopy time and frame rate per procedure, and the mean scatter dose rate for a non-pulsed fluoroscopy mode.

Distance is an important factor that could increase (or decrease) the scatter dose rate. A distance of 65 cm between the cardiologist and the isocentre has been supposed, but a variation of 15 cm nearer to the patient could increase the occupational dose by 70%. In addition, considering that the protective screens—typically equivalent to a shielding of 0.5–1.0 mm lead—can reduce the scatter dose by a factor of 100 if properly used, differences in the reported occupational doses in the scientific literature of two orders of magnitude measured over the lead apron are not surprising. In fact, Pratt and Shaw showed that the relationships between the cardiologist's eye dose and factors such as the dose efficiency of the X-ray equipment, scattered-dose rates, examination protocols and workload are complex and vary from centre to centre [21].

Data considered reliable are scarce in Tables 1 and 2 because between 1989 and 1996, a significant number of cardiologists did not use the personal dosemeters during all procedures or overlooked the established procedure of sending the dosemeters to the medical physics service monthly. Compliance with the radiation badge policies is one of the main problems in many interventional cardiology services. Reported occupational dose values are often surprisingly low and the reason is not a high level of radiation protection, but a lack of use of personal dosemeters. McCormick et al [10] reported that after a mandatory radiation protection training programme, compliance with the radiation badge policy was only 36% in 1999, reaching 67% in 2000 and 77% in 2001 for physicians and nurse clinicians. Therefore, confidence in the mean dose values determined by the regional dosimetry services, and sometimes by the regulatory bodies, to assign occupational doses is open to discussion, as stated by UNSCEAR [5].

Another important point for improving the occupational dosemetry data is reporting dose results from dosemeters over and under the lead apron [1, 13] and combining them to calculate a more realistic effective dose. The over-apron dosemeter provides very useful information on the risk of lens injuries in interventional suites. The data in Table 3 show that differences up to 15% with the conventional under-apron dose approach can be found when considering the proposed NCRP formula [13].

Finally, one may wonder if occupational dose values as high as those reported for the period 1989–1993 could be reached with new X-ray systems and radiation protection facilities. Fortunately, the probability is low. Modern interventional cardiology X-ray systems have significantly decreased the radiation level for the patient and, as a consequence, the scatter radiation level. In addition, radiation protection facilities [8, 22], especially ceiling-suspended screens, are in common use, access to advice of medical physics experts is more frequent and dose parameters are included in QA programmes, among other positive factors. However, the workload of cardiologists and the complexity of procedures are increasing, and thus vigilance should be maintained. In the new X-ray laboratories, high levels of scattered dose are still measured and sometimes a surprising lack of training in radiation protection is the cause of avoidable and unjustified occupational over-irradiations.

	Conclusions

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 
Occupational doses measured on specialists who are routinely using their personal dosemeters show that the radiation protection level has significantly improved in the last decade. A reduction in the effective dose by a factor of 10 has been achieved. The most successful action to reduce occupational doses has been training in radiation protection. The use of ceiling-suspended protective screens in a systematic way by the cardiologists and the programme of patient dose reduction were important complementary actions. New X-ray equipment also contributed to further dose reductions, but its relative impact cannot be distinguished from the training effect in this study because of their interdependence. Another significant conclusion is that mean values of the occupational doses in catheterization laboratories could provide an incorrect estimate of the real radiological risk if some specialists are not using their personal dosemeters on a regular basis.

This study was partially funded by the European Commission 5th Framework Programme, Contract DIMOND FIGM-CT-2000-00061, the Spanish Department for Science and Technology (project BFI2003-09434) and the Spanish Nuclear Safety Council. Validation of some results with TLD chips was carried out with experimental equipment partially funded with EC FEDER resources.

	Acknowledgments

 
The authors thank Mercedes Lago for her help in gathering dosimetry data.

Received for publication May 11, 2005. Revision received August 1, 2005. Accepted for publication September 1, 2005.

	References

Top Abstract Introduction Methods and materials Results Discussion Conclusions References

 

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