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Note: Inspired by this news article brought to my notice by Ves Dimov via twitter. [I couldn't link to the actual tweet, because twitter has been behaving oddly today and has eaten quite a few updates from users]

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A 45-year-old man, who has worked as a stone crusher in a quarry for twentytwo years presented with persistent breathlessness and cough.

CT scan of the Thorax was done*

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Coronal and sagittal reformatted images in lung window setting shows emphysematous changes in both lungs. Numerous centriacinar micronodules (1 to 2mm) are seen in all segments of the right lung and the lingular segments of left lobe (not shown). Few larger nodules are also seen in the subpleural regions of the right upper lobe.

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Coronal reformatted CECT image of shows enlarged lymph nodes in the pulmonary hila on both sides. Few calcific specks are seen in the right hilar lymph nodes.

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Axial plain CT image shows “egg-shell” calcification in the right hilar lymph nodes.

Diagnosis: Silicosis.

The principal sources of industrial exposure to silica are free silica in mining, quarrying, and tunneling; stonecutting, polishing, and cleaning monumental masonry; sandblasting and glass manufacturing; and, in foundry work, pottery and porcelain manufacturing, brick lining, boiler scaling, and vitreous enameling. Coal miners are exposed to dusts that contain a mixture of coal, mica, kaolin, and silica in varying proportions. Silicosis and coal worker pneumoconiosis (CWP) are distinct diseases, with differing histologic features resulting from the inhalation of different inorganic dusts. However, the radiographic and high-resolution CT appearances of silicosis and CWP are quite similar, so that the two disease entities cannot be easily or reliably distinguished in individual cases.

Rapid turbulent airflow in the large central airways changes to slow laminar flow in the peripheral small airways. This results in predominant deposition of particles 1–5 m in diameter in and around the respiratory bronchioles, a roughly centrilobular location in the secondary pulmonary lobule. Regional distribution of pneumoconiotic lesions largely depends on the lymphatic clearance of lung. The main driving force for lymphatic flow is pulmonary arterial pressure. Gravity-dependent gradients exist due to the vertical gradient in pulmonary arterial pressure, with subsequent differences in lymphatic flow between the top and bottom of the lungs. Because the main pulmonary artery is inclined to the left, higher blood flow and lymphatic flow occur in the left upper lobe than in the right. Respiratory excursion increases lymphatic flow. Chest wall motion is thought to milk the lymphatic vessels passively but is not uniform. The outward excursion of the lateral chest wall is less than that of the anterior chest wall but more than that of the posterior chest wall. These regional differences in lymphatic flow result in poor clearance of particles from the posterior part of the right upper lung zone. This superoposterior predilection of dust retention has been described in CT studies.

The characteristic radiologic abnormality seen in patients with simple silicosis or CWP consists of small, well-circumscribed nodules that are usually 2–5 mm in diameter but range from 1 to 10 mm, mainly involving the upper and posterior lung zones. Although there is a tendency for the nodules in silicosis to be better defined than those in CWP, this is not always the case. These small nodules indicate the presence of simple or uncomplicated silicosis or CWP. Egg-shell calcifications in hilar and mediastinal lymph nodes are occasionally seen.

* I did not mention high resolution CT (HRCT) of the lungs, because with the current generation of multislice CT scanners, we routinely get isotropic resolution of about 0.3 - 0.6 mm, which is far superior to the textbook definition of HRCT which is outdated (see further reading #4 below).

Further Reading:

1. Luxury’s dark side: stonecutters’ lung disease - International Herald Tribune, March 16, 2009.

2. Imaging of Occupational Lung Disease. RadioGraphics, Nov-Dec 2001. (Free full text article)
3. Pneumoconiosis: Comparison of Imaging and Pathologic Findings. RadioGraphics, Jan-Feb 2006. (Free full text article)
4. High-Resolution CT of the Lungs. AJR, Sep 2001. (Free full text article)
5. Diagnostic Approach to the Patient With Diffuse Lung Disease. Mayo Clin Proc, Nov 2002. (Free pdf)

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