Nipple shadows refer to the silhouettes of the nipples on frontal chest radiographs, which may mimic solitary pulmonary nodules (SPNs).

Epidemiology

Nipple shadows are apparent on ~7.5% (range 3.5-11%) of frontal chest x-rays ¹.

Pathology

It has been proposed by Miller et al. that solitary pulmonary nodules that reach some or all of the following criteria be considered nipple shadows ²:

• bilateral and symmetric
• "fuzzy" margins or radiolucent "halo"
• sharp lateral border and poorly defined medial border (may be present only on PA projections ³)
• nodules are in a characteristic position:
  • male: between the 5^th and 6^th ribs anteriorly
  • female: at the inferior aspect of the breast shadow
• were not present on a very recent film
• prominent nipples may be visible on a lateral projection

However, if there is doubt whether a nodular opacity represents a nipple shadow or not, a repeat chest x-ray with nipple markers should be performed ⁴, albeit at a financial cost and further radiation dose to the patient.

There is always the rare occurrence of a real solitary pulmonary nodule being overshadowed by a nipple shadow.

Radiographic features

Plain radiograph

On chest radiograph the following features are present:

• nipple shadows are nodular opacities that are consistent in shape, size and position ^1,2:
  • oval or round
  • 5-15 mm in diameter
  • between the 9^th and 10^th ribs posteriorly or the 5^th and 6^th ribs anteriorly
• tend to have incomplete margins due to their attachment to the chest wall ⁴

Differential diagnosis

• solitary pulmonary nodule
  • coin lesion
• pleural or extrapleural lesions ³