Pleural effusion: the thorn sign. Not a rare finding.

Eleven percent of an unselected series of 157 patients with pleural effusion showed a thorn-like protrusion of fluid at the lateral end of the minor fissure at some time during their illnesses. None of a series of 146 normal x-ray films of young patients showed this "thorn sign." The thorn sign is a useful radiographic finding of right-sided pleural effusion.