The term combination shadow refers to a plain film appearance that is the result of a combination density of two separate anatomical structures. It could be argued that all plain film images are combination shadows in as much as three dimension anatomy is represented in two dimensions. The term combination shadow tends to be reserved for structures that are the result of combined opacity that could represent pathology or be otherwise unexplained or confusing

	This 68 year old male presented to the Emergency Department with a history of presyncopal episodes after exertion. He was referred for chest radiography. There are sternal wires associated with a previous thoracotomy.
	There are prominent serratus muscle opacities (white arrows) producing a false impression of pleural effusion on the left (black arrow). There is right sided pleural opacity- probably pleural thickening.
	The lateral projection image demonstrates loss of clarity of the right hemidiaphragm (silhouette sign). The arrowed opacity is not an interlobar effusion- it is a combination shadow produced by the combined opacity of the cardiac shadow and the right hemidiaphragm
	Note that the combination shadow is produced by the combined opacity of the right hemidiaphragm and the posterior cardiac shadow which is within the left hemithorax.