Lung nodules come from a wide range of sources, and the vast majority are not cancer. They’re small masses of tissue, usually less than 3 centimeters across, that show up on chest CT scans. About 27% to 30% of people who get a CT scan of their chest will have at least one, often without any symptoms at all. Infections, both past and present, are the single most common origin, but nodules can also form from inflammation, inhaled particles, benign growths, blood vessel abnormalities, and cancer.
Infections: The Most Common Source
The majority of lung nodules are granulomas, which are small clumps of immune cells that the body builds to wall off an infection. When bacteria or fungi enter the lungs, white blood cells surround the invader and form a tight ball of tissue. Even after the infection clears, that ball often remains as a small scar visible on imaging for years or even a lifetime.
The organisms most frequently responsible are tuberculosis bacteria, nontuberculous mycobacteria, and several types of fungi. Histoplasmosis, caused by a fungus common in the Ohio and Mississippi River valleys, is one of the most frequent culprits in the United States. Coccidioidomycosis (valley fever) plays a similar role in the Southwest. Cryptococcus and blastomycosis are other fungal infections that leave behind nodular scars. Many people who have these granulomas never realized they were infected in the first place, because their immune system handled it quietly.
In people with healthy immune systems, these granulomas tend to calcify over time, developing visible calcium deposits that make them easy to distinguish from cancer on a scan. Nodules that are densely calcified in a characteristic pattern are almost always benign leftovers from old infections.
Inflammatory and Autoimmune Diseases
Certain conditions cause the immune system to create nodules in the lungs even without an infection present. Sarcoidosis is the most well-known example. In sarcoidosis, the immune system forms granulomas in the lungs (and sometimes other organs) in response to an unknown trigger, possibly the body’s own proteins. The typical pattern involves clusters of immune cells accumulating in lung tissue, driven by what appears to be a misdirected immune response to specific antigens.
Rheumatoid arthritis can also produce lung nodules. These rheumatoid nodules are made of inflammatory tissue and tend to appear in people who already have joint disease, though occasionally the lung nodules show up first. Other autoimmune conditions that can produce nodules include granulomatosis with polyangiitis, a disease affecting blood vessels and the lungs, and eosinophilic granulomatosis with polyangiitis, which involves allergic inflammation.
Inhaled Substances and Occupational Exposures
Breathing in certain dusts and particles over months or years can produce nodules in the lungs. Silica dust, encountered in mining, stone cutting, and glass manufacturing, causes a condition called silicosis. The lungs respond to the tiny sharp particles by forming small nodules of scar tissue, often in the upper portions of the lungs. Coal dust causes a similar pattern in coal workers.
Asbestos fibers trigger a different but related process, leading to scarring and sometimes rounded areas of thickened tissue on the lung’s surface. Organic dusts from moldy hay, bird droppings, and certain chemicals can cause hypersensitivity pneumonitis, an allergic lung reaction that produces inflammation and, over time, small nodules and scarring. Even cotton and hemp processing dust can cause lung disease with nodular changes.
Benign Tumors
Not all growths in the lung are cancer. The most common benign lung tumor is a hamartoma, sometimes called a chondroid hamartoma. These are disorganized clumps of normal tissue types that ended up in the wrong arrangement. A typical hamartoma contains a mix of cartilage, fat, connective tissue, and muscle cells. They grow slowly, rarely cause symptoms, and don’t spread to other parts of the body. On a CT scan, the presence of fat or popcorn-like calcification inside the nodule is a strong clue that it’s a hamartoma rather than something more concerning.
Other rare benign growths include lipomas (fatty tumors) and papillomas, but these account for a very small fraction of lung nodules.
Cancer: Primary and Metastatic
About 40% of lung nodules that undergo further evaluation turn out to be cancerous. That number sounds high, but it’s skewed by the fact that nodules with suspicious features are the ones most likely to be tested. The overall rate among all detected nodules, including the many tiny ones that never need workup, is much lower.
Cancerous nodules fall into two categories. Primary lung cancer starts in the lung tissue itself, most commonly as adenocarcinoma. Metastatic nodules, on the other hand, are cancer cells that traveled to the lungs from somewhere else in the body. Cancers of the breast, colon, kidney, and skin (melanoma) are among the most common sources of lung metastases. Distinguishing between a new primary lung cancer and a metastasis from a known cancer elsewhere can be genuinely difficult, sometimes requiring genetic testing of the tissue to determine where the cells originated.
Blood Vessel Abnormalities
A less common but important source of lung nodules is pulmonary arteriovenous malformation, an abnormal connection between an artery and a vein in the lung. These are usually congenital, resulting from a failure of the tiny capillary walls to develop properly during fetal growth. On imaging, they can look like a solid nodule, though they often have a distinctive appearance with visible blood vessels feeding into them.
Most cases are linked to hereditary hemorrhagic telangiectasia, a genetic condition that causes abnormal blood vessel formation throughout the body. In rare instances, chest trauma or liver disease can also produce these malformations. Recognizing them matters because they carry risks of their own, including bleeding and stroke, and they require different management than other types of nodules.
How Size and Appearance Guide Next Steps
When a nodule shows up incidentally on a scan, doctors use its size, shape, and density to estimate how likely it is to be something serious. Nodules smaller than 6 millimeters have a cancer risk below 1%, and for low-risk patients, no follow-up imaging is needed at all. Nodules between 6 and 8 millimeters typically call for a repeat CT scan in 6 to 12 months to check for growth, with another scan around 18 to 24 months if the first follow-up looks stable.
Nodules larger than 8 millimeters get more attention. At that size, options include a repeat scan at 3 months, a PET scan (which detects metabolically active tissue and can help distinguish cancer from inactive scar tissue), or a biopsy. The choice depends on how suspicious the nodule looks, whether you have risk factors for lung cancer such as smoking history, and your own preferences after discussing the options. PET scans are most useful when the estimated chance of cancer falls in the low-to-moderate range, roughly 5% to 65%. If the probability is very high, doctors may recommend going straight to biopsy or surgery.
Part-solid nodules, those with both hazy and solid-looking areas, get their own set of timelines. A ground-glass nodule 6 millimeters or larger that persists on follow-up imaging may be monitored with scans every two years for up to five years, because these slow-growing lesions can represent very early-stage cancers that take years to become a threat.
Multiple nodules are evaluated based on whichever one looks most suspicious, not the total count. Having several small nodules is actually more suggestive of a benign cause like prior infection than a single larger one, though scattered nodules in someone with a known cancer elsewhere raise the question of metastatic spread.