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Pleural cyst of the mediastinum.

Pleural (mesothelial) cyst is an uncommon developmental anomaly of the parietal pleura (Clough and Beirne, 1955). It usually arises from the parietal pericardium, typically in the right cardiophrenic angle (Drash and Hyer, 1950). Less frequently, it is found on the surface of the diaphragm (Clough and Beirne, 1955; Freedman and Simon, 1936). Rarely does a pleural cyst arise from the parietal pleural covering of the upper mediastinum and paravertebral areas and present radiographically as a mediastinal mass (Jamplis et al., 1963). Only two cases have been reported in which it arises from the posterior mediastinum (Jamplis et al., 1963; Robbins, 1943). Pleural cysts are currently not included in the differential diagnosis tables for posterior mediastinal masses in standard reference texts (Fraser and Pare, 1970; Reeder and Felson, 1975).