British Journal ofIndustrial Medicine 1989;46:777-781

Role of computed tomography in evaluating asbestosrelated lung diseaseS LOZEWICZ,' R H REZNEK,2 M HERDMAN,' J E DACIE,2 A McLEAN,2 R J DAVIES'

From the Departments ofRespiratory Medicine,' and Radiology,2 St Bartholomew's Hospital, London EC]7BE, UK

ABSTRACT To find how computed tomography (CT) may be effectively used in individuals withsuspected asbestos related lung disease 30 men with a history ofexposure to asbestos were studied. Allsubjects underwent high kilovoltage posteroanterior and left lateral chest radiographs and chest CT.Eighteen were randomly selected asbestos workers referred for routine surveillance. The remaining 12were patients who had been referred for investigation of respiratory symptoms or abnormal routinechest radiograph, or both, and found to have chest radiographic changes compatible with asbestosrelated lung disease. In the group referred for routine surveillance both pleural shadowing andpulmonary shadowing were shown on CT but not chest radiographs in only one case. Five were

thought to have pleural shadowing on chest radiographs but this was confirmed on CT in only one

case. All 12 patients referred for investigation showed pleural shadowing on chest radiographs; thiswas confirmed in all cases on CT which also showed unsuspected pulmonary shadowing in five cases.

These findings suggest that it is not appropriate to use chest CT routinely in all asbestos workersreferred for routine surveillance. When CT is used selectively in those with pleural shadowing on plainchest radiography, however, it is helpful in refuting or confirming the presence of pleural disease andmay show unsuspected pulmonary shadowing.

Computed tomography (CT) is more sensitive andmore specific than plain chest radiography in showingboth pleural and pulmonary lesions associated withexposure to asbestos.'-7 The role of CT in evaluatingasbestos related lung disease is not defined, however.The purpose of the present study was to find out howCT scanning may be used effectively in individualswith suspected asbestos related lung disease. In par-ticular we aimed to assess its role in the routine medicalsurveillance of asbestos workers referred for examina-tion under the Control of Asbestos at Work Regula-tions 1987, for which the minimum radiologicalexamination currently required is a full size postero-anterior (PA) chest radiograph.8

Population and methods

We studied 30 men of whom 18 were randomlyselected asbestos workers who had been referred forroutine surveillance under the Control of Asbestos atWork Regulations. The remaining 12 were patients

Accepted 19 December 1988

who had been referred to the department of res-piratory medicine at St Bartholomew's Hospital forinvestigation of respiratory symptoms or abnormalitydetected on routine chest radiograph; they wereselected for this study on the basis of their having ahistory ofoccupational exposure to asbestos and chestradiograph changes compatible with asbestos relatedlung disease.The presence or absence of the following symptoms

was recorded; breathlessness walking on level ground,cough on most days, and sputum production on mostdays for as much as three months each year. Detailswere also recorded of their occupational and cigarettesmoking history. All subjects underwent physicalexamination of the respiratory system and lung func-tion tests which included forced expiratory volume inone second (FEV,), vital capacity (VC), total lungcapacity by helium dilution (TLC), and measurementof the transfer coefficient for carbon monoxide.A high kilovoltage PA and left lateral chest

radiographs were performed in all cases. Right and leftanterior oblique chest radiographs were done in 12 ofthe 18 workers referred for routine surveillance and infive of the 12 patients referred for investigations.

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778All subjects had chest CT with a GE9800 whole

body scanner. Scans were made at 1 cm intervalscontiguously from the lung apices to the bases. Allsubjects were scanned in the supine position and nineunselected subjects were additionally scanned prone tolook for changes in gravity dependent perfusion,which have been described in asbestos related lungdisease.23 Subjects were also scanned in the proneposition when shadowing was shown in dependentparts of the lung to determine whether the shadowingwas gravity dependent. All the chest radiographs wereinterpreted by the same experienced radiologist (JED).All the scans were interpreted independently by twoother radiologists (RR, AM) experienced in wholebody scanning and who had no knowledge ofthe chestradiograph appearances. The radiologists were awarethat the subjects had been exposed to asbestos but theydid not know to which of the two subgroups thesubjects belonged.

Results

Subject details and results of lung function tests arerecorded in table 1. None of the subjects had fingerclubbing or inspiratory crackles at the lung bases.

Table 2 shows the numbers of those patientsreferred for investigation who showed changes com-patible with asbestos related lung disease on chestradiographs or CT, or both. Pleural thickening wasTable 1 Subject details

Asbestos exposedAsbestos workers patients withundergoing routine abnormal chestsurveillance radiograph(n= 18) (n= 12)

Age:Mean 31 59Range 14-52 45-78

Time since first exposure (y):Mean 7 33Range 1-30 18-64

Duration of exposure (y):Mean 7 14Range 01-30 0-25-30

Cigarette smoking:Current 10 6Ex 1 6Never 7 0

Respiratory symptoms: 6 9any or all ofbreathlessness, cough,or sputumLung function as mean% predicted (SEM)FEV, 106 (2-43) 65 (4-25)VC 106 (2-39) 73 (5-97)TLC 101 (3.38) 80 (6 06)KCO 87 (3 38) 99 (6.06)

VC = Vital capacity. TLC = Total lung capacity by heliumdilution.KCO = Transfer coefficient for carbon monoxide.

Lozewicz, Reznek, Herdman, Dacie, McLean, Davies

Table 2 Radiologicalfindings in 12 patients exposed toasbestos with chest radiograph abnormalities compatible withasbestos related lung disease

No with shadows

Pleural Pulmonary

Chest radiograph 12 2(PA and left lateral) (all confirmed (both confirmed

on CT) on CT)CTscan 12 7

(all shown on (2 shown onchest radiograph) chest radiograph)

seen in all 12 patients on both chest radiographs andCT. Asbestosis was thought to be present on chestradiographs in only two patients and in each case thiswas confirmed on CT. CT also showed pulmonaryshadowing compatible with asbestosis in a further fiveof the 12 patients (see fig 1). Five of this group hadright and left oblique chest radiographs and in two ofthese the oblique films showed pulmonary shadowingthat was confirmed on CT but not evident on the PAor left lateral chest radiograph. In one of the five,oblique films failed to show pulmonary shadowingthat was evident on CT.

Table 3 shows the numbers of asbestos workersreferred for routine surveillance who showed changescompatible with asbestos related lung disease on chestradiograph or CT, or both. The chest radiograph wasthought to show pleural shadows in five workers butonly one of these showed pleural thickening on CT. Inview of the accuracy with which pleural thickening isseen on CT it is probable that the apparent pleuralshadows on chest radiograph in four of these caseswere false positive findings. In two of these four CTshowed subpleural fat at a site corresponding to theradiograph appearances ofpleural thickening. Pleuralthickening was shown on CT in two workers and inone of these two pleural shadowing was also seen onchest radiograph. Therefore, pleural thickening wasshown on CT but not chest radiograph in only one ofthis group. Furthermore, pulmonary shadowing com-patible with asbestosis was shown on CT in only oneworker (fig 2). Right and left oblique chest

Table 3 Radiologicalfindings in 18 asbestos workersreferredfor routine surveillance

No with shadows

Pleural Pulmonary

Chest radiograph 5 1(PA and left lateral) (one confirmed (not confirmed

on CT) on CT)Computed tomography 2 1

(1 shown on (not shown onchest radiograph) chest radiograph)

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Role ofcomputed tomography in evaluating asbestos related lung disease

©9Fig 1 Chest radiograph (a) in this subject was reported to show pleural shadowing in right lower zone. Lungs were considerednormal. CT shown on soft tissue settings (b) confirmedpresence ofpleural thickening (arrows) with subpleuralfat. In supine(c) andprone (d) positions unsuspected interstitial shadowing compatible with asbestosis was seen when viewed on lungsettings.

radiographs were performed in 12 of this group andthey added no additional information to that alreadyobtained with the PA and left lateral films.

In the group of 30 subjects considered as a whole 14had pleural thickening evident on CT; the pleuralchanges were bilateral in 11. Of the three withunilateral pleural thickening, one had a histologicallyproved mesothelioma and two had additional bilateralpulmonary shadowing compatible with asbestosis. In

no case was pleural shadowing explicable on the basisofprevious infection or trauma. Eighteen subjects hadprone as well as supine scans and none ofthese showedabnormalities in gravity dependent perfusion. Table 4compares the details of the subjects showing scanevidence ofpleural or pulmonary asbestos related lungdisease, or both, with those whose scan was normal;although the 14 subjects in the group with pleural or

parenchymal shadowing, or both, had significantly

Table 4 Clinicalfindings in 30 subjects exposed to asbestos

CTpleural or pulmonaryCT normal (n = 16) shadowing or both (n = 14) p Value

Time since first known occupational exposure (y) (SEM) 6-0 (1-67) 30 8 (4-23) <0 0001Duration of exposure (y) (SEM) 5-58 (1.72) 16-3 (3-17) <0 01Respiratory symptoms: any or all of breathlessness, cough, or sputum. 6 (38) 9 (64) NS(No(%))Lung ftunction % predicted (SEM):FEV, 106 (2-7) 73 (5 50) <0-0001VC 107 (2.52) 79 (5.59) <0-0001TLC 101 (3-85) 83 (5-48) <0 05KCO 87 (3-83) 97(5 34) NS

VC, TLC, KCO, see table 1.

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Lozewicz, Reznek, Herdman, Dacie, McLean, Davies..r .I %

Fig 2 PA chest rtaldiograiph (a.) was reported asnormal. (T.shown htere ont lung set tings itn supine(h} and prone c/c position.s showed unsuspectedinterstiaial shadowing compatible with ashestosvis.

CO-.

( < 0 05) smaller lung volumes, seven had valueswithin two standard deviations ofpredicted and wouldtherefore be assessed as within the normal range.

Discussion

In the group of 18 asbestos workers referred forroutine surveillance only one showed pleural shadow-ing on CT which was not seen on chest radiograph.Similarly only one showed pulmonary shadowing onCT which was not seen on chest radiograph. The cost/benefit ratio for the routine use of CT in this group istherefore likely to be high, and the results suggest thatit is not appropriate to use scanning in all workersreferred for routine surveillance under the Control ofAsbestos at Work Regulations. High resolution CT(HRCT) has recently been found to be more sensitivethan conventional CT in detecting both pleural andpulmonary shadowing in patients with asbestosis9;however, the difference in detection rates was notsubstantial and HRCT was less effective at showingdiffuse pleural thickening, probably because of limita-tions imposed by the sampling technique required in

HRCT scanning. It is unlikely, therefore, that HRCTwould be more useful than conventional CT in theroutine surveillance of asbestos workers.CT showed that four of the 18 workers referred for

routine surveillance had false positive shadowing onchest radiograph (table 3). In two of these four, scansshowed subpleural fat at a site corresponding to theradiograph appearances of pleural thickening. Otherstudies have shown the value of scans in eliminatingfalse positive diagnoses of asbestos related pleuraldisease.'35610 In all 12 patients referred for investi-gation scanning confirmed the presence of pleuralshadowing seen on chest radiograph. These findingstaken together indicate that when scanning is usedselectively in those with pleural shadowing on chestradiograph, it is useful in refuting or confirming thepresence of pleural disease.The 12 patients referred for investigation all showed

pleural shadowing on chest radiographs but only twoshowed pulmonary shadowing, which was seen on CTin seven cases. This suggests that when patientsexposed to asbestos are found to have pleural shadow-ing on chest radiograph, CT often shows unsuspected

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Role ofcomputed tomography in evaluating asbestos related lung diseasepulmonary shadowing. These findings may not applyto workers screened under the current regulations whoare likely to have had a shorter period of exposure toasbestos (table 1) and who are more likely to have usedprotective measures, since their exposure began morerecently (table 1).

Gravity dependent increased density visible on CTmay be due to a combination of distended bloodvessels and lung from which air has been resorbed9; ithas been shown at the lung bases in approximately30% of non-asbestos exposed subjects scanned supinebut disappears in the prone position.9 We routinelyperformed prone as well as supine scans in those withpulmonary shadowing to exclude the possibility thatthe observed changes were gravity dependent. Kreel2and Katz and Kreel3 have described loss of the normaleffect of gravity in causing increased perfusion independent parts of the lung in some patients withasbestos related pulmonary shadowing, sometimes inthe absence of other evidence of interstitial fibrosis; itwas proposed that this alteration in gravity dependentperfusion might be due to some loss of the pulmonarycapillary bed as a result of interstitial fibrosis. Wecould find no abnormalities in gravity dependentperfusion in 18 subjects, including eight with pulmon-ary shadowing, who had prone as well as supine CT.The additional use of oblique chest radiographs can

increase the frequency of demonstration of asbestosrelated abnormalities when compared with PA filmsalone." '3 We obtained oblique chest radiographs in 12of the group referred for routine surveillance and theyadded no additional information to that already withPA and left lateral films; furthermore, they did notshow pulmonary shadowing in the one subject inwhom this was proved on CT. In the group referred forinvestigation oblique radiographs were obtained infive patients and two of these showed pulmonaryshadowing that was not evident on PA or left lateralfilms. One of these five, however, had pulmonaryshadowing on CT that was not evident on oblique,PA, or left lateral chest radiographs. These resultssuggest that CT is more sensitive than plain chestradiography, including oblique films, in detectingpulmonary shadowing.

In the group of 30 subjects considered as a wholethere was a significant association between thepresence of CT pleural or pulmonary shadowing orboth and low lung volumes. In half the subjects withabnormal CT, however, all lung function measure-

ments including lung volumes and transfer coefficientfor carbon monoxide were within two standard devia-tions of the predicted value and would therefore bejudged normal. The presence of normal lung functionin an individual does not therefore preclude thepossibility of CT showing asbestos related lung dis-ease.

This study shows that it is not justified to use CTroutinely in the medical surveillance of asbestosworkers referred under the Control of Asbestos atWork Regulations 1987. Nevertheless, when CT isused selectively in those with pleural shadowing onplain chest radiography, it is helpful in refuting orconfirming the presence of pleural disease and mayshow unsuspected pulmonary shadowing.

References

1 Friedman AC, Fiel SB, Fisher MS, Radecki PD, Lev-Toaff AS,Caroline DF. Asbestos-related pleural disease and asbestosis: acomparison of CT and chest radiography. Am J Radiol1988;150:269-75.

2 Kreel L. Computer tomography in the evaluation of pulmonaryasbestosis. Preliminary experience with the EMI general pur-pose scanner. Acta Radiologica: Diagnosis 1976;17:405-12.

3 Katz D, Kreel L. Computed tomography in pulmonary asbestosis.Clin Radiol 1979;30:207-13.

4 Raithel HJ, Valentin H. Computertomographische Unter-suchungen bei Patienten mit Asbestose und Silikose. Prax KlinPneumol 1983;37:1 119-29.

5 Fourio M, Dongay G, Levade M, Carles P, Bollinelli R, Putois J.Apport de la tomodensitometrie dans la pathologie pleuro-pulmonaire de l'amiante. J Radiol 1984;65:335-9.

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7 Maffessanti M, De Zotti R. Lo studio radiologico delle placchepleuriche nell'asbestosi. Radiol Med 1984;70:825-9.

8 Control ofasbestos at work regulations. London: HMSO, 1987.9 Aberle DR, Gamsu G, Ray CS, Feuerstein IM. Asbestos-related

pleural and parenchymal fibrosis: detection with high-resolution CT. Radiology 1988;166:729-34.

10 Leipner N, Brecht G, Holle JP, Lackner K, Ehlenz P, MagnussenH. Asbestose. Computertomographie im vergleich mit derkonventionellen rontgendiagnostik. Fortschr Geb Rontgenstr1984;141:275-984.

11 Reger RB, Ames RG, Merchant JA, et al. The detection ofthoracic abnormalities using posterior-anterior (PA) vs PA andoblique roentgenograms. Chest 1982;81:290-5.

12 Bgin R, Boctor M, Bergeron D, et al. Radiographic assessment ofpleuropulmonary disease in asbestos workers: posteroanterior,four view films, and computed tomograms of the thorax. Br JInd Med 1984;41:373-83.

13 Fletcher DE, Edge JR. The early radiological changes in pulmon-ary and pleural asbestosis. Clin Radiol 1970;21:355-65.

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