Progressive massive fibrosis (PMF) refers to the coalescence of large nodules. It is more common in silicosis than in coal workers\' pneumoconiosis.

Imaging findings: Mass-like opacities are seen typically in the posterior upper lobes and are associated with the contraction of upper lobes and hilar elevation. On sequential evaluation of these masses, there is migration towards the hila with a peripheral rim of cicatricial emphysema. The outer margins of PMF often parallel the contour of the adjacent chest wall. Large lesions (>5cms) show necrosis and central cavitation (often associated with silico-tuberculosis). These masses may have foci of amorphous calcification. Diameter of masses > 1cm. Usually bilateral, right > left. Overall profusion of nodules decreases due to aggregation into PMF.

Loss of lung volume and peripheral emphysema help to distinguish PMF from neoplastic processes.

Slilca is more fibrogenic than coal. Inhaled silica dust or silicon dioxide (SiO2 ) is deposited in respiratory bronchioles and removed by macrophages and lymphatics. This removal is a slow process. Half life of a single dust burden is 100 days.

Typical occupations: sandblasting, quarries, mining, glass blowing, pottery.

Risk is related to both intensity and duration of exposure. This usually takes > 20 years of exposure. Silicosis is progressive even after removal of dust.

Coal mines also usually contain silica.

Gross pathology: Silicosis primarily involves the upper lung zones. PMF results in end stage lung. Silicotic lung content is 2 - 3%. Normal dried lung silica content is 0.1%.

Microscopic features: Silica particles are centred within concentric lamellae of collagen located along bronchioles, small vessels and lymphatics. On polarised microscopy, silicate crystals are birefringent (1 - 3 microns).