Round (helical) atelectasis.

Round (helical) atelectasis is a little-known form of pulmonary collapse. It is thought to occur secondary to lung compression from pleural effusion or following therapeutic pneumothorax. Its occurrence is favoured in patients with exudative pleural effusions and extensive pleural thickening. It presents radiographically as a pulmonary pseudotumour, and experience with this entity and its pathogenesis are discussed.