Lung nodules can be cancerous, though most lung nodules are noncancerous (benign).
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Lung nodules — small masses of tissue in the lung — are quite common. They appear as round, white shadows on a chest X-ray or computerized tomography (CT) scan.
Lung nodules are usually about 0.2 inch (5 millimeters) to 1.2 inches (30 millimeters) in size. A larger lung nodule, such as one that's 30 millimeters or larger, is more likely to be cancerous than is a smaller lung nodule.
If your doctor detects a lung nodule on an imaging test, it's helpful to compare your current chest X-ray or CT scan with a previous one. If the nodule on earlier images hasn't changed in size, shape or appearance in two years, it's probably noncancerous. In some cases your doctor may recommend annual chest imaging.
Noncancerous lung nodules are often caused by previous infections. Noncancerous lung nodules usually require no treatment. Your doctor may recommend periodic imaging tests to see if a lung nodule grows or changes over time.
If a lung nodule is new or has changed in size, shape or appearance, your doctor may recommend further testing — such as a CT scan, positron emission tomography (PET) scan, bronchoscopy or tissue biopsy — to determine if it is cancerous.
Source: http://www.mayoclinic.org/diseases-conditions/lung-cancer/expert-answers/lung-nodules/faq-20058445/