The round nipple shadow.

It is usually easy to distinguish the round nipple shadow from a lesion, especially by means of repeat radiographs with nipple markers. However, the radiologist must occasionally interpret a round density in the lower chest on the basis of a single radiograph or series of radiographs. Fortunately, a nipple shadow usually has a well-defined outer margin and a poorly defined inner margin. Differentiation of a nipple shadow from an extrapleural or pleural lesion is described. Attention to these findings may prevent needless additional study and patient apprehension.