The natural history and implications of irregularly-shaped small shadows on coalminers’ chest radiographs. Final report on CEC research contract 7248/34/045

Coal workers’ simple pneumoconiosis is diagnosed in life by the presence on a chest radiograph of small opaque shadows. In most cases, these are rounded in shape, with diameter up to about 10 mm. Certain other mineral pneumoconioses are typified by opacities of irregular shape, but many researchers have reported that some coalworkers’ radiographs show some irregular opacities. There has been disagreement about the relevance of such observations to the assessment of occupational disease in coalworkers, particularly since irregular opacities have been observed in older cigarette smokers without substantial occupational exposure to mineral dusts, and many British coalminers smoke cigarettes.This study extends earlier work on irregular opacities, based on radiographs and data collected during the British coal industry’s Pneumoconiosis Field Research (PFR) programme. Subjects studied were selected from men who had attended all of the first four of the (roughly) quinquennial surveys on which the research programme was based, and at least one subsequent survey. A total of 3669 radiographs were studied, from the records of 707 men.The radiographs were examined and classified independently, according to the ILO (1980) Classification scheme, by each of five experienced Medical Officers from the coal industry. These readers worked independently, and examined and classified initially the series of the first four films from each man. Films from the later surveys were then presented and classified, and where this led to revision of a reader’s opinion of any of the earlier films, the revised classification was recorded. In practice, there were few revisions, and they had no effect on the conclusions. The data from the quintuplicate readings of the radiographs numbered over 18,300. These were combined with other data collected at the time of the surveys, and with detailed information on the individual workers’ exposures to respirable coalmine dusts, and were subjected to extensive statistical analyses. The specific research questions were as follows:Ql. Does the percentage of all small opacities which are classified as irregular vary over the 12-point scale of profusion;Q2. Is the anatomical distribution in the lung of predominantly irregular opacities for any one category of profusion similar to that of predominantly roundedopacities with the same profusion category?;Q3. Do the two types of predominant shapes indicate differences in(a) dust-related progression rates?(b) risks of developing progressive massive fibrosis?(c) standardised lung function decrements?(d) cause-specific mortality?;Q4. Are there regional variations in the occurrence of irregularly shaped shadows, and to what extent might these be explicable in terms of different types and compositions of dust?In addition, some limited work was done towards answering the subsidiary question:Q5. To what extent were films, regarded now as showing predominantly irregular opacities, classified in earlier film reading exercises as showing small rounded opacities? And conversely?Despite their common background, the different readers showed considerable disagreement between their opinions of the same films, concerning shape, size, profusion and even presence or absence of small opacities. This lack of unanimity made it difficult to arrive at unequivocal answers to some of the above questions, because in some cases different readers were producing conflicting answers. However, there were common trends in respect of some of the questions, and a certain amount of generalisation was possible.All five readers were in broad agreement over Question 1; for each reader, the proportion of films for which the opacities were described as predominantly irregular decreased with increasing category of profusion of small opacities. It is concluded that past reading practices which may have failed to classify irregular opacities as pneumoconiotic are unlikely to have seriously underestimated the prevalence of simple pneumoconiosis of category 2/1 or more, on which current dust standards are based. The limited work on Question 5, involving comparisons with readings made soon after the films were taken, supported this conclusion; many of the films which were described as predominantly irregular had in fact been classified as showing opacities at the earlier occasion.With regard to Question 2, readers classified the majority of films with opacities as bilateral, but where unilateral involvement was recorded there was unexplained disagreement on which side of the lung was more likely to show opacities. The readers were unanimous in finding higher proportions of irregular opacities in middle and lower zones of the lungs, whereas rounded opacities were observed more often in the middle and upper zones.On Question 3, there was no evidence of any effect of shape of opacities on the risk of progressing to a higher category of profusion, or on the risk of developing the large opacities typical of progressive massive fibrosis (PMF). Four of the five readers classified some film series as showing evidence of PMF against a background of predominantly irregular opacities on the immediately preceding film. There was little evidence of any relationship between lung function decline and shape of opacities; and a similar lack of evidence of any effect of shape of opacities on subsequent cause-specific mortality. The latter finding was based on a relatively short follow-up period and small numbers of deaths, and hence had little power to detect small differences.All the readers showed similar and statistically significant relationships between the prevalence of irregular opacities and estimates of exposure to coalmine dust; but addressing Question 4, there remained unexplained differences between the prevalences at different collieries. These differences were difficult to interpret, since they did not follow the same rank order for the different readers, but there was no evidence that they were in any way related to the proportions of quartz in the dusts from the different collieries.The principal conclusion is that, on present evidence, it is not of major importance to distinguish small rounded from small irregular opacities in epidemiological studies of coalminers’ lung diseases; and that estimates from historical studies of the risk of advanced simple pneumoconiosis of Category 2 or more are not seriously deficient as a result of being based on rounded opacities only. “”