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