The  apical cap is a term used to describe a localized area of soft tissue opacity,  at the apex (top) of one or both lungs, as seen on a chest X-ray or CT scan. It appears as a cap-like density over the uppermost portion of the lung, conforming to the shape of the lung apex which can be concerning and often prompts further evaluation.

An apical cap in the lungs is a radiological finding characterized by a soft tissue density at the apex of the lungs
Cause
Blunt aortic trauma or injury can cause bleeding and hematoma formation near the lung apex, resulting in an acute apical cap.
The apical cap is also associated with chronic inflammatory processes. Common causes include old infections (e.g., tuberculosis), prior inflammatory disease (e.g., sarcoidosis), aging-related fibrosis, or prior trauma and scarring. Other potential causes include chronic exposure to environmental factors, or conditions like emphysema leading to tissue remodeling.
Result
The apical cap in acute blunt trauma indicates aortic trauma which is life threatening and would warrant evaluation with CTA of the aorta. In a chronic setting, it is generally benign and significant clinical consequences are uncommon.
Diagnosis Based on Radiology
On chest X-rays or CT scans, an apical cap typically appears as a soft tissue density, at the lung apex. Clinical history, and stability over time are key in diagnosis.

Causes of an Apical Cap

Apical caps can have various causes, and may arise from the pleural space or from an extra-pleural location

Causes include:

1. Fibrosis or scarring from prior infections or inflammatory conditions.
2. Aging-related changes where fibrosis accumulates naturally with age.
3. Blebs or bullae — air-filled spaces that develop in the lung tissue, commonly associated with emphysema.
4. Pancoast tumors — rare tumors at the lung apex, typically causing additional symptoms like shoulder or arm pain.
5. Post-radiation changes if the area has previously received radiotherapy.
6. Trauma particulalrly of the aorta and apical hemorrhage

Clinical Significance

• Apical caps are usually benign and asymptomatic, especially when seen in elderly patients.
• However, when there are other symptoms (e.g., pain, swelling, or neurological signs), it might warrant further imaging, such as CT or MRI, to rule out malignancy or other serious underlying conditions.

In summary, an apical cap is a localized area of increased opacity at the lung apex on imaging, often due to benign causes but potentially warranting further investigation if accompanied by concerning symptoms or findings.