members: P. A. Gevenois (Belgium), K. G. Hering, T. Kraus, S. Tuengerthal ... H. Itoh, Y. Kusaka, H. Shida, N. Suganuma (Japan), M. Crane (UK), D. A. Henry and ...
Is CT screening for asbestos-related diseases rational ? Narufumi Suganuma, M.D.1 Yukinori Kusaka, M.D.1 Harumi Itoh, M.D.2 1 Division of Environmental Health, Department of International, Social and Health Sciences, University of Fukui School of Medicine, Fukui 910-1193, Japan 2 Department of Radiology, University of Fukui School of Medicine, Fukui 910-1193, Japan
Key words: high-resolution CT, spiral CT, classification, non-malignant asbestos-related diseases, malignant asbestos-related diseases
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Computed tomography is a more sensitive and specific radiological examination than chest radiograph for detecting asbestos-related diseases. Especially, targeting lung cancer, CT may benefit more than chest radiograph, though mortality reduction by the CT screening program has not yet been proven. Asbestos Expert Meeting held in Helsinki in 2000 was yet to recommend CT screening for lung cancer, but agreed to develop international scheme to classify HRCT findings of asbestos-related diseases. Consequently, an international collaboration study including experts from 7 countries resulted publication of the International Classification of HRCT for Occupational Environmental Respiratory Diseases (ICOERD). The ICOERD classification is designated to evaluate HRCT findings of non-malignant asbestos-related diseases. It is supposed to be used with spiral CT for screening malignant diseases. In order to minimize the radiation to the subjects, low dose technique should be applied to both HRCT and spiral CT. Newly available multi-detector CT may be used to obtain both gap-less thick section and thin section CT for evaluating malignant and non-malignant asbestos-related diseases. The protocol that was used in our pilot CT screening program is discussed in this session and proposal is made based on the experience.
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INTRODUCTION Computed tomography is a more sensitive and specific radiological examination than chest radiograph for detecting asbestos-related diseases. Especially, targeting lung cancer, CT may benefit more than chest radiograph for detecting lung cancer in early stage(1, 2), though mortality reduction by the CT screening program has not yet been proven(3). Theme of this paper is to discuss whether CT screening among asbestos exposed current and past workers is rationale. In order to examine this issue it is needed to clarify the target disease of the screening and technical parameters demanded for the purpose. Asbestos Expert Meeting held in Table 1 Target disease and technologic demands for Helsinki in 2000 did discussed these matters the CT screening Target disease Technologic demands and summarized that for the malignant diseases gap-less spiral CT is needed to be Malignant diseases - Lung cancer low dose spiral CT performed, while for the non-malignant - Mesothelioma asbestos related diseases high-resolution CT Non-malignant diseases high-resolution CT is necessary for evaluation of early findings - Asbestosis ILO-like coding system - Pleural thickening (Table 1). The meeting concluded that it would yet to recommend spiral CT screening Table 2 Imaging parameters for HRCT screening Collimation: Iess than mm for lung cancer among asbestos-exposed Reconstruction algorithm: high-spatial workers, but agreed that it may useful in the frequency or çsharpé algorithm Scan tme: 1-2 seconds near future after thorough evaluation on 120kV, 40-150 mAs mortality reduction was done. The participants WW/W: 1500-2000/-300to-700, 150350/20-50 also agreed to develop international scheme Matrix size: 512 x 512 to classify HRCT findings of non-malignant Prone if possible ** International expert meeting on new advances in the radiology asbestos-related diseases and set prototype and screening of asbestos-related diseases. Scand J Work Environ Health 2000;26(5):449-454. of technological guideline (4). Summary of the agreed imaging parameter is shown in Table 2. Relatively wide range for recommended window settings is a result of international negotiation. An international collaboration study including experts from 7 countries that comprised from the Helsinki meeting resulted publication of the International Classification of HRCT for Occupational Environmental Respiratory Diseases (ICOERD)(5, 6). The ICOERD classification is designated to evaluate HRCT findings of non-malignant asbestos-related diseases. HRCT screening for non-malignant asbestos-related diseases is supposed to be used with spiral CT for screening malignant diseases. In order to minimize the radiation to the subjects, low dose technique should be applied to both HRCT and spiral CT. Newly available ● ●
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multi-detector CT may be used to obtain both gap-less thick section and thin section CT for evaluating malignant and non-malignant asbestos-related diseases. The protocol that was used in our pilot CT screening program is discussed in this session and proposal is made based on the experience.
Thin section CT findings of non-malignant asbestos related diseases
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Thin section CT enables us to examine earliest findings of non-malignant asbestosrelated diseases that appear in secondary lobule of lung parenchyma. Subpleural dots appear usually on or off the posterior pleura at interlobular septum. This seems one of the earliest signs of asbestosis, but this sign only is not sufficient enough to diagnose asbestosis and Finnish Fibrosis Grading System, one of the prototypes of ICOERD, considers it as borderline manifestation(7). Intralobular core lines or centrilobular opacities are also considered earliest finding of asbestosis which is pathologically correlated with peribronchiolar fibrosis(8), but this sign can be found among smokers (RB-ILD) and hypersensitivity pneumonitis. Interlobular septal thickening, which correlated to pathologic interlobular fibrotic thickening, is highly indicative of interstitial fibrosis and when this findings profuse more, indicator of severity such as traction bronchiectasis and/or honeycombing would appear
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together. Subpleural curvilinear line is considered to be a merged intralobular core lines, and specific finding of asbestosis (Fig 1). These earliest HRCT manifestations are well described with pathological correlation in Akiraís reports(8, 9). Parenchymal band is a specific finding in asbestosis because it implies coexistence of pleural abnormalities. The HRCT classification considers this finding as an indicator of visceral pleural thickening. Parietal pleural thickening or plaque is also specific findings of asbestos-related respiratory diseases (Fig 2). It should be noted that there are two normal findings that mimics these abnormal findings: depending opacities that mimic interstitial fibrosis in the posterior part when supine scan and fat pad
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that looks similar to plaque on chest radiograph, but easily distinguished by HRCT. The guideline for clinical diagnosis of non-malignant asbestos related diseases issued by American Thoracic Society (ATS) in 2004 accepts imaging as equivalent to histology in structural evidence of the diseases(10).
Spiral CT screening for lung cancer Although HRCT or thin section CT can detect very early fibrotic findings, gap-less scan of whole lung with thin section CT may not be feasible. One of the target diseases in CT screening among asbestos exposed workers is lung cancer, and gap-less scan is inevitable
CT screening among the asbestos exposed Radiation dose reduction is inevitable for screening among healthy workers and ex-workers, as CT scan usually have radiation dose of 10mGy with 200mAs. Spiral CT screening in general population performed in Japan uses 25-50mAs low dose CT. There are some study based screening program performed in Finland(11, 12), Germany(13) and Japan targeting lung cancer and non-malignant asbestos-related diseases among asbestos exposed present workers and ex-workers. These studies used standardized HRCT classification scheme to classify asbestos-related non-malignant diseases, and used spiral CT for detecting lung cancer. We have done pilot screening program among 100 construction workers who were
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for the purpose. New technology of multi-detector CT enable to obtain whole lung scan with thinsection and to reconstruct images into appropriate thickness thereafter (Fig 3), but the apparatus is not available all around the world. When we speak of CT screening among asbestos-exposed workers, we have to consider whether it is for malignant disease that should be done with gap-less scan or it is for non-malignant disease that should be done with thin-section scan.
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selected from 3000 members for a construction union. Inclusion criteria for the screening program were volunteers who match 1) 50 years and older, 2) smokers or ex-smokers; and 3) asbestos exposure in house or building construction. CT screening was done by 16 channels MDCT with low dose protocol with 25mAs, which gives radiation dose of 2 to 3mGy. The reconstruction of obtained images was done as 7mm thickness for lung cancer screening and 2mm thickness for non-malignant asbestos related diseases. Three cases of lung cancer, two of them were confirmed by pathologic diagnosis, 6 small nodules that need follow-up study were detected. As for non-malignant asbestos-related diseases prevalence of fibrotic findings was roughly doubled compared to radiographic asbestosis. It is also similar in the prevalence of pleural plaque. Limited results of our study seemed to encourage us to perform CT screening among asbestos exposed workers, using certain inclusion criteria on asbestos exposure, age and smoking habits. Kraus et al proposed a multiplicative risk assessment for lung cancer among asbestos exposed workers in order to select participants of CT screening program(14). Few countries use HRCT or spiral CT for medical screening of asbestos-related diseases. In Japan, legitimate medical screening for pneumoconioses using chest radiograph and lung function test among workers or ex-workers is supplemented by spiral CT, especially in case physician suspects possibility of malignant complications. Pneumoconioses includes silicosis, coal workers pneumoconiosis, asbestosis and other rare dust-induced pulmonary fibrotic diseases in Japanese Pneumoconiosis Law that regulate the contents of medical screening for pneumoconioses. The lawís recommendation of applying spiral CT is based on the idea that profused small opacities may prevent nodules to be detected. CT is legitimately accepted as a tool for medical screening for lung cancer but not for non-malignant asbestos related diseases now in Japan. But in France HRCT screening for asbestos related diseases is performed. In Germany, ICOERD is used as official classification system for evaluation of HRCT of asbestos related diseases(13). Should we implement CT screening program where the current chest radiograph screening and surveillance program for asbestos related diseases have low coverage for the population at risk of asbestos-related diseases? Our answer is negative. As for now mortality reduction effect by introducing spiral CT screening for lung cancer is yet to be proven. Also the cost of the CT apparatus and cost per scan are both extremely high compared to chest radiograph, undergoing CT scan need more skilled radiological technicians than taking chest radiographs. CT apparatus are less in number than radiographies, interpretation is more difficult in CT than in chest radiograph. Introduction of sophisticated modality of screening and surveillance is not the only key to reduce asbestos-related diseases, but integrated program of medical screening and technological preventive measures with high coverage of
population at risk will fulfill the task. In these countries radiographic screening should be the primary choice for secondary prevention of asbestos related diseases. Digital radiography may reduce In countries where lung cancer screening trials among general populations are performed now, screening for lung cancer plus non-malignant asbestos related disease among asbestos exposed workers, especially 50 years and older, is acceptable. Low dose technique is highly recommended in order to reduce radiation dose. Smoking cessation should be encouraged to asbestos-exposed workers. The test interval should be carefully decided in order to find more lung cancer and to reduce radiation dose.
Conclusion CT screening among asbestos exposed workers is acceptable when low dose technique of 25-50mAs is used and targeted for lung cancer and non-malignant asbestos related diseases. Its implementation should be limited to the countries where radiographic screening and surveillance is already established and have good coverage of the population at risk. In order to eliminate asbestos-related diseases integrated intervention of medical and technological prevention is needed, rather than merely introducing new modalities of medical screening. Full coverage of population at risk of the diseases by the integrated preventive measures is inevitable.
Acknowledgement
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The CT Classification Study Group that developed ICOERD consists of following members: P. A. Gevenois (Belgium), K. G. Hering, T. Kraus, S. Tuengerthal (Germany), L. Kivisaari, T. Vehmas (Finland), M. Letourneux (France), M. Akira, H. Arakawa, Y. Hosoda, H. Itoh, Y. Kusaka, H. Shida, N. Suganuma (Japan), M. Crane (UK), D. A. Henry and J. E. Parker (USA).
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