The chest x-ray shows
a cavity near the right cardiophrenic angle.
A CT scan done to characterise the cavity showed an uncomplicated pericardial cyst near the lower right heart border.
CAVITY is a useful mnemonic to remember causes of a pulmonary cavity (as per Radiopedia)
Vascular (pulmonary emboli)
Infections (staphylococcus, klebsiella, tuberculosis, aspergillosis)
Trauma – pneumatocele
Youth (pulmonary sequestration, bronchogenic cyst, pericardial cyst)