An apical cap is a caplike lesion at the lung apex, usually caused by intrapulmonary and pleural fibrosis pulling down extrapleural fat or possibly by chronic ischemia resulting in hyaline plaque formation on the visceral pleura. The prevalence increases with age. It can also be seen in hematoma resulting from aortic rupture or in other fluid collection associated with infection or tumor, either outside the parietal pleura or loculated within the pleural space. [from
HPO]