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Q1: What do the chest radiograph and HRCT of the lung show (see p 628)?
The chest radiograph shows nodular opacities, predominantly in the upper lobes. The HRCT lung images show diffusely distributed centrilobular nodules without any evidence of fibrosis.
Q2: What is the differential diagnosis of the HRCT scan appearance and the likely diagnosis in this case?
The differential diagnosis of centrilobular nodules with diffuse distribution on HRCT lung scan (see p 628) are infectious bronchiolitis including tuberculosis, pneumoconiosis (coal worker’s pneumoconiosis and siderosis), vasculitis and vascular metastases, sarcoidosis, hypersensitivity pneumonitis, diffuse panbronchiolitis, and respiratory bronchiolitis-interstitial lung disease (box 1). The diagnosis in this case was siderosis in view of the clinical presentation, occupational history of prolonged exposure to iron oxide during silver polishing, and characteristic appearance of centrilobular nodules on HRCT of the lungs.
Box 1: Differential diagnosis of centrilobular nodules on HRCT lung scan
Infectious bronchiolitis including tuberculosis.
Pneumoconiosis (coal worker’s pneumoconiosis and siderosis).
Vasculitis and vascular metastases.
Respiratory bronchiolitis-interstitial lung disease.
Siderosis (synonyms: welder’s lung, buffer’s lung, or silver polisher’s lung) is a non-fibrogenic or a “benign” form of pneumoconiosis due to the inhalation of iron particles. Iron dust is an inorganic, inert, mineral dust with high radiodensity, which neither causes substantial proliferation of reticulin fibres nor gives rise to collagenous fibrosis when retained in the lungs.1 Occupations leading to siderosis involve exposure to iron oxide dust or fumes and include silver and steel polishing, iron and steel rolling, steel grinding, electric arc welding, fettling, stripping and dressing castings in iron foundries, boiler scaling, and mining iron ores.1
Patients are usually asymptomatic unless there is concurrent smoking or contamination of air with other chemicals such as silica or asbestos.2 They may have a reddish coloured sputum due to exposure to these dusts.1 Siderosis is therefore essentially a “radiological disorder”, due to the presence of very radiodense opacities, but with no functional impairment.2,3 Iron oxide exposure may be carcinogenic for the human lung.4,5 The emission of polycyclic aromatic hydrocarbons as pyrolysis products of organic materials used may be responsible, but requires further confirmation.
The pathology of siderosis is characterised by perivascular and peribronchiolar aggregation of dark pigmented iron oxide particles present extracellularly in alveolar spaces and walls as well as in macrophages.1 Slight reticulin proliferation may be present in siderosis, but there is no collagenous fibrosis. If fibrosis is present, it is secondary to the presence of crystalline silica.
Radiologically, siderosis presents as centrilobular opacities on HRCT with a uniform distribution throughout the lung fields with no conglomeration. Centrilobular opacities on HRCT lung can be divided into two types according to size.6 The larger ones are seen as sharply demarcated, rounded nodules and these correspond to the q and r types of pneumoconiosis (box 2) seen on the chest radiograph.7 The smaller ones, more frequent in number, are seen as relatively ill defined nodular or branching opacities, a few closely spaced dots, or areas of low attenuation on HRCT and represent radiographic type p pneumoconiosis. The nodules are present diffusely and bilaterally, but with upper lobe and posterior predominance.
Box 2: International Labor Office classification of radiographs of pneumoconiosis7
Round opacities are classified according to size as: p, q, or r (p, up to 1.5 mm in diameter; q, 1.5 ∼ 3 mm; r, 3 ∼ 10 mm).
Irregular opacities are classified as: s, t, and u (fine, medium, or coarse respectively).
Combination of round and irregular as: x, y, and z.
Siderosis, baritosis, and stannosis are types of benign pneumoconiosis with radiographic dense opacities.
Siderosis, also called welder’s lung, buffer’s lung, or silver polisher’s lung is the most common type.
Usually asymptomatic, respiratory symptoms may be present in smokers or if there is concurrent exposure to silica or asbestos.
Centrilobular nodules with diffuse distribution are seen on a HRCT lung scan.
Diagnosis is made on radiological features and occupational exposure.
The differential diagnosis of centrilobular opacities on HRCT lung includes miliary tuberculosis, pneumoconiosis, metastases, sarcoidosis, hypersensitivity pneumonitis, panbronchiolitis, and respiratory bronchiolitis-interstitial lung disease. Some radiographic features are characteristic of the underlying disorder. In miliary tuberculosis, the size of nodules is relatively uniform throughout the lungs. In haematogenous metastases, the metastatic nodules are usually smooth, well defined, and round in shape. They are usually variable in size and do not show ill defined, fine centrilobular nodular or branching opacities. A classical perilymphatic distribution of the nodules is seen in conditions like silicosis, sarcoidosis, lymphangitic metastases, and amyloidosis.
Patients with siderosis require no treatment and the radiological changes of siderosis may regress after cessation of exposure.3 The diagnosis of siderosis should be considered in relevant occupations with characteristic radiological abnormalities and absence of respiratory symptoms.
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