The aortopulmonary (AP) window is a key review area on chest x-rays as it can hide potentially serious pathology.

The AP window normally has a concave lateral border. A straight lateral border can be considered normal if unchanged from the previous chest x-ray. A newly straightened or convex lateral border is considered abnormal and has a number of causes:

• mediastinal lymphadenopathy (most common)
• prominent mediastinal fat (normal variant)
• aortic or bronchial artery aneurysms
• malignancy, e.g. nerve sheath tumor