Pulmonary nodule

Last revised by Liz Silverstone on 14 Mar 2024

Pulmonary nodules are small, rounded opacities within the pulmonary interstitium. Pulmonary nodules are common and, as the spatial resolution of CT scanners has increased, detection of smaller and smaller nodules has occurred, which are more often an incidental finding.

Pulmonary nodules can be classified according to size, morphology and/or distribution.

They are generally homogeneous (without air bronchograms or alveolograms) and are well-defined since their margins are sharp and they are surrounded by normally aerated lung parenchyma. They are quite separate from airspace nodules that often have an irregular margin and are usually ~8 mm in diameter. (For further discussion, see the article on nodular opacification.)

The differential diagnosis for a nodule can be refined by its size, location, and density. Solitary pulmonary nodules and hyperdense pulmonary nodules are discussed further elsewhere.

A micronodular or miliary pattern is predominately seen in granulomatous processes, hematogenous pulmonary metastases, and pneumoconioses. Nodules and masses are most often seen in metastatic disease to the lung.

Always be aware of artifacts, especially on radiographs, where buttons or nipple shadows can often be mistaken for a true pulmonary nodule 7

The term micronodule has been defined variably by different position statements. In the National Lung Screening Trial (NLST) all nodules with <4 mm maximum diameter were recorded as micronodules 8. Some authors consider lesions measuring <3 mm as micronodules 9. In this article, we follow the cutoff recommended by the Fleischner Glossary published in 2024 which classifies nodules <6 mm as micronodules, which are generally ‘non-actionable’ 11

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