Diffuse

Types: Circumscribed (plaques), and/or diffuse (Note 12).

Site: Pleural thickening of the chest wall is recorded separately for the right and left chest walls.

Width: For pleural thickening seen along the lateral chest wall the measurement of the maximum width of the shadow is made from the inner line of the chest wall to the inner margin of the shadow seen most sharply at the parenchymal-pleural boundary. The maximum width usually occurs at the inner margin of the rib shadow at its outermost point.

a = maximum width up to about 5 mm.

b = maximum width over about 5 mm and up to about 10 mm.

c = maximum width over about 10 mm.

For pleural thickening seen face on (en face), the presence is recorded, even if it can also be seen in profile. If pleural thickening is seen face on only, width usually can not be measured.

Extent: Extent is defined in terms of the maximum length of pleural involvement, or as the sum of maximum lengths, whether seen in profile or face on.

1 = total length equivalent to up to one quarter of the projection of the lateral chest wall.

2 = total length exceeding one quarter but not one half of the projection of the lateral chest wall.

3 = total length exceeding one half of the projection of the lateral chest wall.

Note 12 Pleural Thickening

Shadows are produced by thickening of the visceral and parietal layers of the pleura. Confident identification of which surface is thickened may not always be possible using the postero-anterior radiograph. But separation may be useful because there may be differing etiologies, natural histories and relations to disability.

Pleural thickening occurs in two principal forms, the different appearances of which are illustrated in the standard radiographs. Both types may occur together. Separation of the two types is usually, but not always possible. In some cases small peripheral shadows of indeterminate type occur.

(a) Circumscribed shadows (non-calcified hyaline plaques)

The standard radiographs include an example of the characteristic appearance of these shadows. They are normally the result of thickening of the parietal pleura, and are the precursors of calcified plaques.

The extent of these shadows in relation to the length of the lateral chest wall provides a rough measure of their size. But a postero-anterior radiograph may reveal only a small proportion of their full extent. The width of the shadow measured between the inner edge and the chest wall is highly influenced by the precise position of the plaque on the chest wall, so that this measurement may not be and indication of severity.

(b) Diffuse shadows

The standard radiographs include examples of the appearances of these shadows. This is the well recognized type of pleural thickening seen also in many non-pneumoconiotic conditions. It is probably the result of thickening of the visceral pleura. It is less specific to asbestos and other fibrous mineral dust exposure, but it is a feature in some cases of severe asbestosis. The word 'diffuse' refers to the tendency to produce a general veiling of lung parenchymal detail. In some cases the shadow produces a sharply defined line along the chest wall due to it being seen in profile (edge on). When this appearance is well developed the maximum width of the shadow is recordable. The extent is recorded as for circumscribed shadows. These two measurements may provide the best index of the severity of visceral pleural involvement.