While posterior subcapsular cataracts are characteristic of radiation exposure, several sets of data suggest that the broader category of posterior cortical cataracts may also be regarded as radiation-associated. Increased risks of lens opacities (including posterior subcapsular, cortical, nuclear, and mixed cataracts) have been reported in different populations for the lower doses induced by chronic, fractionated, or acute exposure to γ or neutron radiation, β particles, galactic cosmic radiation, and X-rays (Table 1). An increased frequency of radiation-induced lens opacities has also been observed in a variety of environmental, medical, and occupational contexts. Reports of lens opacities related to environmental exposure come from the Hiroshima and Nagasaki survivors 5 6 7 , from children living in the contaminated territories of Chernobyl 8 , and residents of (60)Co-contaminated buildings in Taiwan 9 . Sources of medical exposure include X-Ray exposure during computed tomography 10 and radiotherapy 11 12 . Occupational exposure to IR and lens opacities have been reported for Chernobyl clean-up workers 13 , astronauts 14 15 16 and pilots 17 , and medical personnel, such as radiology technicians 18 . Nonetheless, new data from exposed human populations are still necessary to confirm the absence of a dose threshold, or the need to revise the existing threshold.

The widespread use of IR in medical practice for both diagnostic and therapeutic purposes results in a significant increase in exposure of both patients and medical staff 19 . The use of medical imaging involving X-rays as a diagnostic tool or during interventional procedures has increased steadily over the last few years, particularly in the field of interventional cardiology, including cardiac electrophysiology 20 21 . Interventional cardiologists are now thought to be the most highly exposed of all medical personel 22 23 24 . The ablation of atrial fibrillation performed by electrophysiologists, for example, is a long and potentially irradiating procedure 25 , and the operator's eyes are exposed to scattered X-rays. The frequent failure of some cardiologists to use protective leaded eyewear helps explain the crucial need for radiation monitoring and risk assessment for medical staff 26 . Interventional cardiologists are exposed to risks in the same range as those for which early-stage cataracts have been observed.

Very few epidemiological studies have been published on the risk of cataracts in interventional radiology (including procedures for interventional cardiology). Junk et al. 27 took a first step towards identifying and increasing awareness of these risks in a study that screened 59 volunteer participants at a professional meeting of interventional radiologists, including cardiologists. They observed a surprisingly high frequency of posterior subcapsular cataracts in their sample: 22 individuals (37.3%) had small paracentral dot-like opacities in the posterior subcapsular region of the lens, consistent with early signs of radiation damage, and five more (8%) had diagnoses of cataracts (corresponding to more advanced stages of lens opacities). This study nevertheless had limitations: a selection bias due to their recruitment method, which may have resulted in overestimating prevalence, and the absence of a control group of unexposed participants. A study presented at the European Society of Cardiology congress in 2009 28 did use both an exposed and a control group and found a significant difference in the frequency of lens opacities (37.9% vs. 12%, p < 0.005). These findings reinforced the results reported by Junk et al. Nevertheless, as the authors underlined, their study also had methodological limitations, including the same potential selection bias described for Junk et al. and an age difference between exposed and unexposed group that may partly account for the results (46.7 vs. 40.5 years). A recent study examined the prevalence of radiation-associated posterior lens opacities among 56 interventional cardiologists and 22 controls: 52% of the former had lens opacities and only 9% of the latter 29 . Overall, relatively few cataracts have been reported among medical staff in interventional radiology, although some authors have stressed the lack of adequate monitoring 30 .

In conclusion, a few studies have examined cataracts among interventional cardiologists, but the cause of the early cataracts identified has not been completely investigated. An epidemiological study in this population should provide further knowledge about the potential risk of radiation-induced cataracts in populations with exposure levels thus far considered to be low and should also improve cardiologists' awareness of the need for radiation protection. For these reasons, the Institut de Radioprotection et de Sûreté Nucléaire, with the cooperation of interventional cardiology groups of the French Society of Cardiology, has launched the O'CLOC study (Occupational Cataracts and Lens Opacities in Interventional Cardiology), designed to test the existence of an increased risk of cataracts among interventional cardiologists, compared with a control group of cardiologists not exposed to X-rays.

The aim of the O'CLOC study is to compare the prevalence of different stages and types of cataracts (from no opacities to severe cataracts; nuclear, cortical or posterior subcapsular) in an exposed group of interventional cardiologists with an unexposed reference group of non-interventional cardiologists, while taking into account other risk factors for cataracts, use of radiation protection tools, and exposure level.

There are approximately 1700 interventional cardiologists in France: 1000 coronary interventional cardiologists (CICs) and 700 cardiologists specializing in the treatment of cardiac arrhythmias (arrhythmologists or electrophysiologists). Stratification of recruitment according to this distribution of CICs and electrophysiologists make this study the first to study both types of cardiologists exposed to X-Rays. It should therefore be as representative as possible - at least in terms of proportion - of interventional cardiology in France.

The relative youth of the population of cardiologists has presented difficulties previously in studying cataracts in this group, specifically, the absence of data about lens opacities either in the general population for the same age class as cardiologists who could be considered as unexposed (most data concern essentially "senile cataracts"), or in an appropriate control group. That is, information about the background frequency of lens opacities in a reference population is essential. This study will compare a group of exposed individuals (interventional cardiologists chronically exposed to X-rays) to a group of unexposed but otherwise comparable individuals: cardiologists not occupationally exposed to ionizing radiation (non-interventional cardiologists). The O'CLOC design is presented in Figure 1. French centres employing CICs and arrhythmologists will be selected according to several criteria: employment of at least two interventional cardiologists; balanced distribution of the centres across France; a balanced distribution of public and private hospitals. All cardiologists in the selected centres will be contacted and invited to participate. Only cardiologists at least 40 years old will be included. This age criterion was chosen to ensure occupational exposure to IR for at least 10 years in the exposed group. Subjects with a history of personal medical radiation exposure (radiotherapy, brain scans) will be excluded from both groups. Moreover, non-interventional cardiologists with a cumulative significant history of work in interventional cardiology above one year will also be excluded. To ensure comparability between the exposed and unexposed groups, subjects will be matched by sex and age.

The non-interventional cardiologists included in the study are considered unexposed. A retrospective evaluation of the IR exposure status of the interventional cardiologists will be necessary. We will assign an exposure category level to each interventional cardiologist based on the information collected in the occupational questionnaire and from the SISERI system, as presented above. It will take into account: specialization in interventional cardiology (CICs or electophysiologists), the questionnaire information (we will consider at a minimum: duration of practice, duration of exposure and the numbers performed of each type of procedure, but other variables, such as the use of protection, handedness (left or right), equipment (film or digital), etc., will also be taken into account when possible, data from the literature (quantifying the cumulative number of examinations per physician and extrapolation from literature data about the mean dose for each type of procedure at various points in time, to estimate the total dose received), and information from the SISERI system (as presented above).

To estimate the cataract risk associated with exposure, the analysis will first compare the prevalence of all types and stages of cataracts (e.g., any type/no opacities; any type/any stage; any type/stage1; any type/stage2 cortical/no opacities; cortical/stage1; etc.) between the exposed and unexposed groups. Second, a sensitivity and specificity analysis will further analyze the outcomes to estimate the relative risk of the different types and stages of cataracts associated with interventional cardiology practices, with adjustments for matching variables (age, sex), but also for potential confounders (e.g., myopia and diabetes). Finally, the retrospective evaluation of potential exposure will be used for a qualitative study of the dose-response relation. This analysis, to the extent possible, will be based on exposure levels, will be adjusted for confounders, and will use the unexposed group as the reference group.

Participant recruitment began in October 2009 and is planned to continue through January 2011. As of April 2010, 135 cardiologists (105 interventional and 30 non-interventional) have been recruited. The results should be available by 2011 and we will publish our findings, whether they are positive, negative, significant, or not significant.