A lung nodule is defined as a spot on the lung that is 3 cm in diameter or less. If an abnormality is seen on an X-ray of the lungs that is large than 3 cm, it is considered a lung mass instead of a nodule, and is more likely to be cancerous.
Lung nodules usually need to be at least 1 cm in size before they can be seen on a chest X-ray. Lung nodules are quite common, and are found on 1 in 500 chest X-rays and 1 in 100 CT scans of the chest. Approximately 150,000 lung nodules are detected in people in the U.S. each year. Roughly half of smokers over the age of 50 will have nodules on a CT scan of their chest.