Extrapleural sign

Last revised by Frank Gaillard on 28 May 2023

The extrapleural sign refers to the appearance of a pulmonary opacity with oblique margins that taper slowly to the chest wall when the lesion is viewed tangentially to the x-ray beam. This appearance suggests that the lesion is pleural or extrapleural in nature, as opposed to intrapulmonary where an acute angle would be expected as the lesion meets the lung periphery. This term may be confused with extrapleural air sign which refers to a different finding.

The appearance has also been referred to as the snowball sign 4. The analogy for an intrapulmonary mass is a thrown snowball just prior to impact with the (chest) wall, which maintains an acute angle. The pleural or extrapleural mass corresponds to the snowball after impact with the wall, which is flattened and makes obtuse angles with the wall.

History and etymology

The extrapleural sign was described by Ben Felson in 1973 1.

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