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The radiological pattern of pneumoconiosis is changing in recent years.
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The basic pathology in pneumoconiosis is shown.
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The early and advanced stages of metal lung are different.
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The concept of dust-related diffuse fibrosis is recognized with use of HRCT.
Abstract
The radiological patterns of known pneumoconiosis have been changing in recent years. The basic pathology in pneumoconiosis is the presence of dust macules, mixed dust fibrosis, nodules, diffuse interstitial fibrosis, and progressive massive fibrosis. These pathologic changes can coexist in dust-exposed workers. High resolution CT reflects pathological findings in pneumoconiosis and is useful for the diagnosis. Pneumoconiosis such as silicosis, coal workers' pneumoconiosis, graphite pneumoconiosis, and welder's pneumoconiosis, has predominant nodular HRCT pattern. Diffuse interstitial pulmonary fibrosis is sometimes found in the lungs of this pneumoconiosis. In the early stages of metal lung, such as aluminosis and hard metal lung, centrilobular nodules are predominant findings, and in the advanced stages, reticular opacities are predominant findings. The clinician must understand the spectrum of expected imaging patterns related to known dust exposures and novel exposures. In this article, HRCT and pathologic findings of pneumoconiosis with predominant nodular opacities are shown.
Pneumoconiosis is a word coined from the Greek (pneuma = air and konis = dust) and introduced in the 19th century to describe lung diseases caused by the inhalation of mineral dusts.
The International Classification of HRCT for Occupational and Environmental Respiratory Diseases (ICOERD) is increasingly used for scoring the extent of disease seen on CT scans.
International CT Classification Study Group. Selection of reference films based on reliability assessment of a classification of high-resolution computed tomography for pneumoconioses.
The history of occupational exposure to dust is the clue to diagnosis of occupational lung disease. However, dust-exposed persons potentially are subject to the same spectrum of lung diseases as the general population. Uncommon pneumoconioses are difficult to diagnose when the parenchymal abnormalities are not well known to radiologists. A combination of clinical features, occupational history, and radiological findings is essential for the diagnosis. The purpose of this article is to present pathological findings and images of a variety of pneumoconioses. High resolution CT imaging of pneumoconiosis with predominant reticular opacities is previously reported.
Inert or weakly fibrogenic dusts usually produce an interstitial aggregate of dust-filled macrophages known as a ‘dust macule’. Fibrogenic dusts can cause hyaline nodules or satellate areas of fibrosis, peribronchiolar fibrosis, diffuse interstitial fibrosis, and large fibrous masses (progressive massive fibrosis). Mixed dust fibrosis (MDF) produces stellate lesions, often with less collagen than in typical silicotic nodules, and the collagen is arranged in randomly or interlacing bundles. MDF occurs at a lower percentage of silica to the total lung dust than when silicotic nodules occur. Diffuse interstitial fibrosis is seen in association with exposures to asbestos, silica, silicates, coal, and some metals. It is important to understand the pathologic reactions to a wide variety of dust particles.
Fig. 1Pathological reactions of the lung to dust. A to D, left to right.
(a) Macules are focal collections of dust-laden macrophages in the walls of respiratory bronchioles with mild and predominantly reticulin fibrosis. (b) Silicotic nodule is a generally sharply circumscribed nodular lesion with a whorled collagenous center. (c) Mixed dust fibrosis is a stellate (medusa head) lesion, often with less collagen than a typical silicotic nodule; the collagen is arranged in interlacing bundles or randomly. (d) Diffuse interstitial fibrosis.
In principle, dust macules are hard to depict or show nodular opacities on radiographs. Silicotic nodules and mixed dust fibrosis usually show nodular opacities on the images. Mixed dust fibrosis sometimes shows reticular opacities.
Diffuse interstitial fibrosis shows reticulation on the images.
In the International Labour Organization (ILO) classification of radiographs of pneumoconioses, small rounded opacities are those with a diameter <10 mm (with subdivisions of 0–1.5 (p), 1.5–3 (q), and 3–10 mm (r)).
International Labor Office Guidelines for the use of the ILO international classification of radiographs of pneumoconiosis. Occupational safety and health series no. 22 (Rev. 2011).
Pneumoconioses with predominant p type opacities include atypical silicosis, coal workers' pneumoconiosis, welders' lung, graphite pneumoconiosis, talcosis, activated carbon pneumoconiosis and aluminosis. The HRCT scans of patients with radiographic type p pneumoconiosis are characterized by tiny branching structures in a centrilobular location, whereas opacities of the q and r types are sharply demarcated, rounded nodules or irregular nodules in the paraseptal and subpleural regions.
Inhaled crystalline silica particles smaller than 5 μm are deposited in the small airways and alveoli from which they are ingested by alveolar macrophages or tissue macrophages within 48 h of deposition, if they penetrate the interstitium. Free particulate silica not ingested by macrophages enters the perivascular lymphatic channels to be translocated to the draining mediastinal lymph nodes as free particles or within macrophages.
There are many types of tissue reaction in silicosis. Two main manifestations of silicosis have been distinguished: acute and chronic with a subset of accelerated chronic occurring in <10 years.
Overlap between these can occur. The classic pulmonary response to crystalline free silica is the formation of hyaline and collagenous nodules in the lung (chronic form). The silicotic nodules are usually observed in centrilobular, paraseptal, and subpleural regions and have a perilymphatic distribution.
The nodules tend to be predominantly located in upper and posterior portions of the lungs. Clearance of particles is poorest in the posterior part, particularly in the upper lung zones because of regional differences in lymphatic function.
Lopes AJ, Mogami R, Capone D, et al. High-resolution computed tomography in silicosis: correlation with chest radiography and pulmonary function tests. J Bras Pneumol 28; 34:264-272.
Ooi et al. reported that the reduced levels of lung function in patients with silicosis was correlated with superimposed emphysema rather than the nodular profusion and that silicosis, in the presence of progressive massive fibrosis (PMF), caused significant emphysema.
The authors did not clearly indicate the type or mechanism of emphysema. Silicosis is associated with pleural abnormalities, including pleural thickening, effusion, and pleural invagination.
Since silicosis is a perilymphatic disease, it is difficult to discriminate between truly pleural disease and involvement of the subpleural interstitium.
In a study, chronic interstitial pneumonia was pathologically confirmed in 19.3% of 233 cases of consecutive silicosis and mixed-dust pneumoconiosis.
Chronic interstitial pneumonia in silicosis and mix-dust pneumoconiosis: its prevalence and comparison of CT findings with idiopathic pulmonary fibrosis.
In this study, 25% showed a pattern not typical of idiopathic pulmonary fibrosis (IPF), while the remaining showed a pattern typical of usual interstitial pneumonia (UIP) pattern. The features of chronic interstitial pneumonia associated with silicosis and mixed-dust pneumoconiosis were subpleural homogeneous attenuation, random distribution of fibrosis, and presence of silicotic nodules. Areas of homogeneous attenuation pathologically corresponded to dense accumulation of collagen and silicotic nodules.
Chronic interstitial pneumonia in silicosis and mix-dust pneumoconiosis: its prevalence and comparison of CT findings with idiopathic pulmonary fibrosis.
Serial HRCT in chronic interstitial pneumonia related to silica exposure showed that the faint ground-glass opacity that was indistinguishable from the non-specific interstitial pneumonia (NSIP) pattern on CT had progressed to honeycomb changes. Honeycombing developed in all patients over a median period of 12.1 years. Available autopsy findings confirmed a diagnosis of a UIP pattern.
The dust-exposed patients with chronic interstitial pneumonia seem to have a better prognosis compared with IPF. The clinical course of IPF is variable, however, the long-term survival in IPF is distinctly poor, being only 20 to 30% survival 5 years after the time of diagnosis without the novel therapy, such as perfenidone.
In addition, it is not clear whether the interstitial lung disease is related to pneumoconiosis or is an additional accompanying disease.
Acute silicosis, also called silicoproteinosis, occurs in subjects exposed to very high concentrations of respirable crystalline silica for a few weeks to 5 years. Sandblasting is the most common cause of acute silicosis.
Pathologically, there is a proteinaceous and cellular alveolar proteinosis. Radiologic findings can be similar to those in alveolar proteinosis. Reported HRCT findings of acute silicosis include multiple ill-defined centrilobular nodules, patchy ground-glass opacity, and lung consolidation in the posterior portions of the lungs. Calcification within areas of consolidation is a common finding. Calcification of hilar or mediastinal lymph nodes is also a common finding.
The ‘crazy-paving pattern’ seen in alveolar proteinosis is seen in some cases. Accelerating silicosis, a subset of chronic form, develops 5–10 years after initial exposure. In accelerated silicosis, the major features of the disease are identical to those of its chronic disease (Fig. 2a, b ). It may also have features typical of acute silicosis in developing alveolar lipoproteinosis.
In addition, accelerated silicosis is associated with the presence of silicotic nodules that develop sooner than in chronic silicosis. The silicotic nodules of accelerated silicosis are usually smaller than the classic silicotic nodules seen in chronic silicosis.
HRCT scans at the early stage (a) demonstrated septal lines reflecting lymphatic dust clearance in addition to nodular opacities diffusely distributed throughout both lung. Serial CT scans obtained at 5 years later (b) demonstrated the progression of the disease from a diffuse linear pattern to a confluence of nodules (arrow). The linear opacities appear thicker than in panel a.
The characteristic lesion of CWP is the collection of closely packed dust-laden macrophages with little collagen (coal dust macule) around a respiratory bronchiole. Such lesions usually are associated with dilatation of respiratory bronchioles, so-called focal emphysema. Another type of lesion is the fibrotic nodules. Collagen in these nodules has a haphazard arrangement of fibers.
PMF is pathologically defined as nodules measuring 1 cm or more in diameter. Mixed dust fibrosis and silicotic nodules are also present in most patients, as the miners are exposed to a dust that is a mixture of coal, kaolin, mica, and silica. “Coal mine dust lung disease” encompasses a variety of interstitial lung disease (coal worker's pneumoconiosis, silicosis, mixed dust pneumoconiosis, and coal mine dust-related diffuse fibrosis) and chronic airway diseases including emphysema and chronic bronchitis.
Antao VC dos S, Petsonk EL, Sokolow LZ, et al. Rapidly progressive coal workers’ pneumoconiosis in the United States: geographic clustering and other factors. Occup Environ Med 205; 62:670-674.
‘Rapidly progressive coal workers’ pneumoconiosis' has previously been defined as an increase in small opacity profusion subcategory >1 in a 5-year span and/or any case of PMF occurring in a miner younger than 70 years of age with <40 years of coal mining tenure. In one study, 277 (35.4%) were cases of rapidly progressive CWP.
Antao VC dos S, Petsonk EL, Sokolow LZ, et al. Rapidly progressive coal workers’ pneumoconiosis in the United States: geographic clustering and other factors. Occup Environ Med 205; 62:670-674.
Another study showed that a total of 163 (84.9%) miners had a normal initial radiograph. The mean time from the most recent normal radiograph to a radiograph consistent with PMF was 20.7 years (median 20; range 1–43). Of these 163 miners, 27 (16.6%) progressed to PMF in <10 years, 57 (35.0%) progressed to PMF in 11–20 years and 79 (48.5%) miners progressed from normal to PMF in >20 years.
Rapidly progressive CWP occurs in younger coal miners and in certain geographic regions that are so-called hot spots, and they are more likely to have worked in small mines and at the coal face.
Antao VC dos S, Petsonk EL, Sokolow LZ, et al. Rapidly progressive coal workers’ pneumoconiosis in the United States: geographic clustering and other factors. Occup Environ Med 205; 62:670-674.
the lung pathology had features of accelerating silicosis, mixed dust pneumoconiosis, and PMF. Four had diffuse interstitial fibrosis with chronic inflammation, and two had focal alveolar proteinosis. Areas of UIP-like interstitial fibrosis were seen in several specimens. Polarized light microscopy revealed large amounts of birefringent mineral dust particles consistent with silica and silicates and carbonaceous coal dust was less prominent. The authors suggested that the rapidly progressive pneumoconiosis in these miners was associated with exposure to coal mine dust containing high concentrations of respirable silica and silicates.
Diffuse interstitial pulmonary fibrosis is sometimes found in the lungs of coal miners. Prevalence of dust-related diffuse fibrosis ranged from 15 to 20% in autopsy studies of coal miners.
Dust-related diffuse fibrosis may manifest as bridging fibrosis connecting to the macular, nodular, or PMF lesions of CWP or silicosis, often showing pigmented interstitial septal thickening.
Among radiographic opacities of CWP, small rounded opacities are commonly described as occurring predominantly in the upper lung zones. A recent study however showed that small radiographic opacities were approximately equally distributed throughout the lung zones.
In this study, mainly rounded opacities were seen in 62.1% of miners and mainly irregular opacities were seen in 37.9%. Miners with primarily rounded opacities had a distribution with moderate upper zone predominance, whereas miners with primarily irregular opacities showed a lower zone predominance.
Surveillance chest radiographs of U.S. miners show an increasing proportion of r-type pneumoconiotic opacities, suggesting greater exposure to silica and silicates.
Radiographic-pathologic correlation in coal miners showed r-sized opacities associated with silicosis and irregular opacities, often more prominent in middle and lower lung zones, as manifestations of coal mine dust lung disease; dust-related diffuse fibrosis and/or emphysema.
Graphite or plumbago is one of the allotropic forms of crystalline carbon. Analytically pure graphite may cause pneumoconiosis although the majority of the pneumoconiosis is a mixed-dust type of lung reaction.
Graphite pneumoconiosis closely resembles coal miners' pneumoconiosis in many respects. The most common CT findings of graphite pneumoconiosis are small nodular opacities and the prevalence of conglomeration, i.e. PMF.
The small nodular opacities are usually in a centrilobular location but also along interlobular septa and in the subpleural regions. Small nodular opacities are classified into two patterns: ill-defined tiny opacities that appear either as fine branching structures or as a few dots clustered together; or well-defined discrete nodules. Ill-defined nodular opacities are abundant, and there are comparably fewer discrete nodules. HRCT-pathologic correlation reveals that the ill-defined small nodules correspond to macular lesions along the walls of bronchioles, which are often dilated, and that the discrete nodules correspond to larger macular or nodular lesions.
Reticular hyperattenuating areas similar to findings in diffuse interstitial pulmonary fibrosis are a predominant finding in some cases with graphite pneumoconiosis.
In two cases of graphite pneumoconiosis with coarse reticulation on the radiograph, common HRCT findings included predominant centrilobular nodules, a few larger nodules, ground-glass opacity, interlobular septal lines, and consolidation with high attenuation in the subpleura or the bronchovascular region predominantly in the lower lungs (Fig. 3a, b ). Lobular low attenuation areas were sometimes seen, as also slightly enlarged lymph nodes with high attenuation. Pathologic examination showed dust macules, interstitial fibrosis with honeycombing, and marked fibrosis with a large amount of graphite dust (Fig. 3c, d).
Fig. 349-Year-old man with a history of exposure to graphite dust 17 years.
(a) HRCT scan shows diffuse ground-glass opacities, irregular consolidated areas, conglomeration, and intralobular interstitial thickening, as well as extensive parenchymal micronodules in areas of visible secondary lobules.
(b) CT scan at mediastinal windows shows peripheral conglomerated mass with areas of high attenuation.
(c) Gross evaluation of the pathology specimen shows black macules. Black macules are found in graphite, coal, carbon, and cigarette smoke.
(d) Photomicrograph of lung specimen obtained from autopsy demonstrates dust macule and marked fibrosis with a large amount of graphite dust (H&E stain).
(e) Photomicrograph of lung specimen obtained from autopsy (H&E stain).
(f) Area of interstitial fibrosis viewed with partially crossed polarizers shows numerous highly birefringent particles of dust.
MDF is the pulmonary response to the simultaneous inhalation of dusts containing free silica and less fibrogenic dusts in some industrial process. Exposure to dusts with high content (>18% of the total dust deposited in lung) of free crystalline silica results in classic silicosis, while mixed dust fibrosis develops in the presence of low silica content (<18% of total dust deposited in lung).
MDF is seen in silicosis, CWP (coal mine dust lung disease), and other pneumoconioses. The pathology is classified as “mixed dust” when mixed dust lesions have a higher score than other lesions, and as “silicosis” when silicotic nodules have a higher score than other lesions.
Mixed dust pneumoconiosis (MDP) is defined pathologically as a pneumoconiosis showing dust macules and/or MDF, with or without silicotic nodules in an individual with a history of exposure to mixed dust.
Small irregular opacities on chest radiographs often proved to have reticular or honeycomb patterns on CT scans. Irregular opacities seen on radiographic and CT images represented interstitial fibrotic or mixed dust fibrotic changes associated with the accumulation of birefringent particles and emphysematous change, as noted at histologic analysis.
In two autopsied cases of MDP studied by us, the common HRCT findings were centrilobular nodules, ground-glass opacities, interlobular septal lines, reticular opacities, emphysematous spaces, and honeycomb cysts. Pathologic examination obtained at autopsy showed macules, MDF, patchy interstitial fibrosis predominantly concentrated around respiratory bronchioles and brown pigmented macrophages, and severe interstitial fibrosis with honeycombing (Fig. 4a–e ). The area of interstitial fibrosis contained numerous highly birefringent particles of dust (Fig. 4f).
Fig. 450-Year-old man with mixed dust pneumoconiosis.
(a) HRCT shows emphysematous spaces and areas of ground-glass attenuation. Centrilobular opacities are also seen in circle area.
(b) HRCT shows emphysematous spaces, areas of ground-glass attenuation, and honeycomb cysts.
(c) Photomicrograph obtained in circle area shows patchy interstitial fibrosis predominantly concentrated around respiratory bronchioles and brown pigmented macrophages. Centrilobular nodular opacities are seen in this circle area.
(d) Photomicrograph obtained in circle area shows severe interstitial fibrosis with honeycombing. Honeycomb cysts are seen in this circle area.
(e) Photomicrograph obtained in circle area shows diffuse interstitial fibrosis and mild chronic inflammation. Areas of ground-glass attenuation are seen in this circle area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Exposure to welding fumes is known to be a risk factor for chronic respiratory disorders – such as pneumoconiosis, chronic obstructive pulmonary disease (COPD), and lung cancer.
The principal component of the inhaled dust is iron-oxide, thus the condition is known as welders' siderosis. The main pathologic features of siderosis are iron dust and dust-laden macrophages associated with little fibrosis (macules). The most common CT findings in arc welders are centrilobular nodules diffusely distributed in the lung, mimicking hypersensitivity pneumonia. The centrilobular nodules do not reflect reactive fibrosis but, rather, radiopaque accumulations of iron particles that lie within macrophages, aggregated along the perivascular and peribronchiolar lymphatic vessels.
Diffuse pulmonary fibrosis has often assumed to be attributed to inhalation exposures other than iron oxide during welding, as a form of mixed dust fibrosis. High-dose exposure to welding fume and inadequate ventilation may also lead to interstitial fibrosis.
It is reported that steel welding fume may cause an occupational respiratory bronchiolitis which may develop into desquamative interstitial pneumonia with continued exposure.
Diffuse pulmonary fibrosis with accumulation of aluminum metal is sometimes seen with chronic exposure to high concentrations of fumes during aluminum arc welding.
The fibrosis is usually mild and its distribution is prominent in the lower lung zones. Centrilobular nodules are also seen in cases of diffuse pulmonary fibrosis. Honeycombing was found in a few cases studied by us. The honeycombing seen at HRCT was either traction bronchiolectasis or combined emphysema and honeycombing. Conglomerated masses with areas of high attenuation are reported in welders' siderosis.
Contrary to aluminosis, the centrilobular opacities of hard metal lung disease are predominantly distributed in the lower lobes (Fig. 5). In the advanced stages of both, HRCT scan shows reticular opacity, traction bronchiectasis, bullae, and honeycombing.
Fig. 5HRCT scans of 32-year-old man with early stage of hard metal pneumoconiosis.
The basic pathology in pneumoconiosis is the presence of dust macules, MDF, nodules, diffuse interstitial fibrosis, and PMF. These pathologic changes can coexist in dust-exposed workers. Pneumoconiosis is a classic disease, but now the radiological pattern of pneumoconiosis is changing in recent years. The concept of dust-related diffuse fibrosis is recognized with the use of HRCT in the evaluation of occupational lung disease.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Our Institutional Review Board approved that informed consent was unnecessary, because this study was retrospective and patient's name was blinded.
Declaration of competing interest
Data sharing is not applicable to this article as to datasets were generated or analyzed during the current study.
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International CT Classification Study Group. Selection of reference films based on reliability assessment of a classification of high-resolution computed tomography for pneumoconioses.
Guidelines for the use of the ILO international classification of radiographs of pneumoconiosis. Occupational safety and health series no. 22 (Rev. 2011).
Lopes AJ, Mogami R, Capone D, et al. High-resolution computed tomography in silicosis: correlation with chest radiography and pulmonary function tests. J Bras Pneumol 28; 34:264-272.
Chronic interstitial pneumonia in silicosis and mix-dust pneumoconiosis: its prevalence and comparison of CT findings with idiopathic pulmonary fibrosis.
Antao VC dos S, Petsonk EL, Sokolow LZ, et al. Rapidly progressive coal workers’ pneumoconiosis in the United States: geographic clustering and other factors. Occup Environ Med 205; 62:670-674.
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