Obstruction of a major bronchus often leads to a combination of atelectasis and retention of secretions with consequent pulmonary opacity, but collateral air drift may partially or completely prevent these postobstructive changes. Secondary infection may occur beyond the obstruction.

The following features suggest that pneumonia is secondary to an obstructing neoplasm:

1. The shape of the collapsed or consolidated lobe may be altered because of the bulk of the underlying tumour. In cases with lobar collapse due to a central tumour mass, the fissure in the region of the mass is unable to move in the usual manner and, therefore, the fissure may show a bulge (the Golden S sign if involving the right upper lobe). The description now seems to have become extended to include the CT equivalent appearance of a hilar mass and collapsed distal lobe and also to describe the phenomenon in other lobes (Fig. 1).

2. The presence of pneumonia in an at-risk patient, confined to one lobe (or more lobes if there is a common bronchus) that persists unchanged for longer than 2–3 weeks, or a pneumonia that recurs in the same lobe, particularly if the lobe shows loss of volume and no air bronchograms. Simple pneumonia often clears or spreads to other segments within a few weeks. In practice, complete resolution of pneumonia virtually excludes an obstructing neoplasm as a cause of infection. Although consolidation may improve partially on appropriate antibiotic therapy, it almost never resolves completely if secondary to an underlying carcinoma. Occasionally, the opacified lobe may appear larger than normal because of the build-up of infected secretions beyond the obstructing carcinoma, an appearance that has been labelled the 'drowned lobe'.

3. A visible mass with irregular stenosis of a mainstem or lobar bronchus. Careful analysis of CT images may demonstrate the presence of an obstructing tumour when there is obstructive atelectasis.

4. Simple pneumonia rarely causes radiographically visible hilar adenopathy, though enlarged central nodes may be seen on CT or MRI. Lung abscess can occasionally be confused with bronchial carcinoma because it may result in hilar or mediastinal adenopathy.

5. Mucus-filled dilated bronchi may be visible within collapsed lobes on a CT examination as branching, tubular low-density structures, and when seen should prompt a search for a centrally obstructing tumour (Fig. 2)

FIGURE 1 ■ Lobar collapse. The tumour in the bronchus intermedius is causing partial middle and lower lobe collapse.

FIGURE 2 ■ Fluid-filled dilated bronchi beyond a central obstructing carcinoma are visible in this collapsed and consolidated right lower lobe.

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