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

Pathology

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

Size

• miliary nodules: <2 mm

• pulmonary micronodule: <6 mm (see: practical points)

• pulmonary nodule: 4-30 mm

• pulmonary mass: >30 mm

Morphology

• solid pulmonary nodules

  • calcified pulmonary nodules

• partly solid pulmonary nodules

• ground glass pulmonary nodules

Distribution

• centrilobular pulmonary nodule

• diffuse pulmonary nodules

• juxtapleural lung nodule

• perilymphatic pulmonary nodule

  • perifissural pulmonary nodule

  • subpleural pulmonary nodule

• random pulmonary nodule

Radiographic features

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.)

Differential diagnosis

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, haematogenous 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. 

Practical points

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. 

See also

• pulmonary mass

• incidental lung nodules

  • British Thoracic Society guidelines for pulmonary nodules

  • Fleischner Society pulmonary nodule recommendations

  • Brock model

• necrobiotic lung nodules

• solitary pulmonary nodule