A lung nodule is a small, solid abnormality in the lung. Nodules are most commonly detected on a chest X-ray or CT scan.

There are many causes of nodules. Depending on the cause, a nodule may have no importance or it may represent a serious disease.

Some of the most common causes of lung nodules include:

• Scar tissue
• Cysts
• Minor vascular abnormalities
• Current or past infection
• Cancer (including lung cancer or cancer in another part of the body that has spread to the lung)

There are many other, rarer causes, including tumors that are not cancerous and autoimmune diseases.

Certain features of lung nodules help doctors decide how serious they are and whether they should be tested (with a biopsy), removed or left alone. For example, if a person with a single, small lung nodule is young, has never smoked and the X-ray shows calcium in the nodule, the nodule is likely to be noncancerous. Doctors may recommend repeating an X-ray or CT scan in a number of months to be sure it's not growing or developing features more suggestive of cancer. On the other hand, the suspicion for cancer will be higher if an older smoker has a larger nodule or if the person has a prior history of cancer. It's important to review old chest X-rays or CT scans to look for changes in the appearance of a lung nodule.

There is often uncertainty about the nature of a lung nodule when it is first detected. Most of the time, decisions about the importance of a lung nodule and the best course of action will come from a biopsy, its response to treatment (such as antibiotics) or by confirming that a nodule is not changing over time.