The main finding in both PA and lateral chest radiographs was very well described by Marcos Mestas. The trachea is displaced to the right in the PA image (blue arrow) and the space behind the trachea, the so called retro tracheal space is occupied. (yellow arrow).

With these findings a suspicion is raised of mediastinal pathology, and CT should follow.

CT (Non contrast due to renal problems): A fluid collection with a thick wall is identified between the trachea and the oesophagus. (red dot).

Putting together the clinical history of recent neck surgery and the findings of a collection in the vicinity of trachea and oesophagus the diagnosis was of a Mediastinal abcess.

You must remember that the Mediastinum is connected to the neck and therefore tumoral and infectious processes may “migrate” downwards thanks to this pathway and to gravity.

This patient was treated with antibiotics and a post treatment CT showed that the collection had disappeared as you can see in the following images.

Points to remember. 

1. Look for subtle findings in the mediastinum in PA and lateral chest radiographs and remember the retro tracheal space.
2. Cervical pathology may extend to the mediastinum helped by gravity.

Here is an excellent reference from my good friends Dr. Tomás Franquet and colleagues.