Updated January 01, 2015.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

If you’ve been told that you have multiple lung nodules after a test like an x-ray or a CT scan, it can be very frightening. What exactly are lung nodules, what are some of the possible causes, and what might you expect as you work with your doctor to determine the cause?

Definition of Multiple Lung Nodules

Lung nodules are defined as “spots” on the lung that are 3cm (about 1 ½ inches) or less in diameter.

If an abnormality seen in the lungs is larger than this, it is called a lung mass. Multiple lung nodules, in contrast to a solitary pulmonary nodule, include the finding of two or more nodules within the lungs.

Causes of Multiple Lung Nodules

Unfortunately, the most common cause of multiple lung nodules is cancer that has spread (metastasized) to the lungs from other regions of the body. That said, there are many benign (non-cancerous) causes of multiple nodules as well. Some possible causes include:

• Malignant (Cancerous) Tumors - As noted above, metastatic cancer from other regions of the body is the most common cause of multiple lung nodules, especially if you have a history of cancer. Nearly any cancer can spread (metastasize) to the lungs. The most common of these include breast cancer, colon cancer, prostate cancer, and bladder cancer.
  Cancers other than lung cancer in the lungs, such as lymphomas, and Kaposi’s sarcoma, may also lead to multiple nodules.
  
  However, multiple lung nodules also may have non-cancerous causes or be due to non-spreading forms of cancer.
  

• Benign Tumors - Such as hamartomas. Hamartomas are benign tumors that are made up of fat, cartilage, and connective tissue -- tissues that normally occur in the lungs, but grow in a disorderly way.
• Autoimmune - Conditions such as Wegeners granulomatosis, sarcoidosis, eosinophilic granules, and rheumatoid nodules can appear as multiple nodules.
• Infections - Several types of infections can result in the appearance of multiple nodules on a scan of the lungs. Some of these include:                                                                      Bacterial infections – septic nodules (spread of infection via the bloodstream from another area of the body), tuberculosis, nocardiosis                                                                                     Fungal infections – aspergillosis, histoplasmosis, coccidiomycosis, cryptococcosis         Parasites – echinococcus (hydatid cysts), paragonimus (liver flukes)
• Inflammatory (pneumoconiosis) – better known as black lung disease (coal miner’s lung), silicosis.

How Common Are Lung Nodules?

Lung nodules are fairly common, being found on 1 in 500 chest x-rays and 1 in 100 CT scans. In one study, 51% of smokers aged 50 and older had lung nodules on CT scans.

What Tests Might My Doctor Recommend?

Since the most common cause of multiple lung nodules is cancer that has spread from other parts of the body, tests may be recommended to look for a primary cancer – that is, a cancer in another part of the body that could have spread to the lungs. Examples of these may include a mammogram to look for a breast tumor, or a colonoscopy to look for a colon tumor.
Your doctor may also recommend tests to further understand the nodules that are in your lungs. These may include CT scans, MRI studies, or a PET scan. A recent small study found that a combination of CT and PET scans helped in determining the cause of multiple nodules more accurately than either of these tests alone. If the cause of your lung nodules is not obvious based on other studies, a biopsy may be done to get a sample of one of the lung nodules in order to determine the cause.

Other tests will depend upon the possible causes of your nodules, for example blood tests may be ordered to evaluate for infectious and inflammatory causes.

What Are the Chances That My Lung Nodules Are Cancer?

As noted above, if you have a history of cancer, the chance that your lung nodules are cancerous is higher. Larger lung nodules -- those greater than 1 cm (or ½ inch) in diameter -- are more likely to be malignant, whereas smaller nodules are more likely to be benign. If you are in good overall health and nodules are discovered incidentally (that is, accidentally) on a study done for some other reason, your nodules are more likely to be benign. Certain descriptions on x-ray studies may also suggest whether or not your nodules are likely to be cancerous. If your nodules are described as “calcified” it is more likely that they have a benign cause. In contrast, nodules described as having a “ground glass” appearance are more likely to be malignant.

• Differences Between Benign and Malignant Tumors

Further Reading:

• All About Lung Nodules – Symptoms, Causes and Diagnosis
• Causes of Lung Nodules
• Solitary Pulmonary Nodules

<sub>Sources:

Chen, T., and M. Gould. Evaluation of patients with small, subcentimeter nodules. Seminars in Respiratory and Critical Care Medicine. 2008. 29(3):241-7.

Gould, M. et al. Evaluation of patients with pulmonary nodules: when is it lung cancer?: ACCP evidence-based clinical practice guidelines (2nd edition). Chest. 2007. 132(3 Suppl):108S-130S.

MacMahon, M. et al. Guidelines for Management of Small Pulmonary Nodules Detected on CT Scans: A Statement from the Fleischner Society. Radiology. 2005. 237:395-400.

Ost, D., and M. Gould. Decision Making in the Patient with Pulmonary Nodules. American Journal of Respiratory and Critical Care Medicine. 2011 Oct 6. (Epub ahead of print).

Travaini, L. et al. Roles of computed tomography and [(18)F]fluorodeoxyglucose-positron emission tomography/computed tomography in the characterization of multiple solitary solid lung nodules. Ecancermedicalscience. 2012. 6:266.</sub>