The unilateral tranradiant hemithorax is rather like the ‘pint half full or half empty’ arguement. Is the transradiant side the abnormal or normal side?
The differential diagnoses include causes that are everything from insignificant to immediately life threatening.

These include:
a. Rotation of the Patient ( the transradiant side is the one the patient is turned towards)
b. Mastectomy
c. Poland’s Syndrome
d. Pneumothorax
e. Pulmonary embolus
f. Unilateral bullae
g. Compensatory emphysema
In this case the cause was simply rotation (see the marked rotation of the film), that caused such a dramatic difference in appearance to the hemithoraces.

Reference: Chapman & Nakielny. Aids to Differential Diagnosis, Saunders, p114-5.

Credit: Dr Ian Bickle