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)

Cancer

Autoimmune

Vascular (pulmonary emboli)

Infections (staphylococcus, klebsiella, tuberculosis, aspergillosis)

Trauma – pneumatocele

Youth (pulmonary sequestration, bronchogenic cyst, pericardial cyst)