Extrapleural sign

The extrapleural sign, described by Ben Felson in 1973 , 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 . 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.