A lung nodule is a small, abnormal spot of dense tissue found in the lung, measuring less than three centimeters in diameter. The discovery of a lung nodule is a common occurrence, as they are found in up to 1% of all computed tomography (CT) scans and 0.2% of chest X-rays. These spots are often found by chance when imaging is done for other medical reasons. While the term can cause concern, it is simply a descriptive word for a finding on an image.
How Lung Nodules are Visualized
The initial detection of a lung nodule often happens with a standard chest X-ray. On an X-ray, a nodule appears as a white, shadowed spot against the darker background of the air-filled lungs. While useful for initial identification, a chest X-ray provides a two-dimensional view, which can sometimes be limiting. Overlapping structures, like ribs, can obscure a nodule, and it can be difficult to determine the precise size and shape from a flat image.
For a more detailed assessment, a computed tomography (CT) scan is the preferred method. A CT scanner takes a series of X-ray images from different angles around the body and uses computer processing to create cross-sectional, or “sliced,” images of the lungs. This technology provides a much clearer, three-dimensional view, allowing for precise measurement of the nodule’s size, shape, and density.
Distinguishing Benign from Malignant Nodules
Radiologists analyze several visual characteristics on CT scans to assess the likelihood of a nodule being cancerous. These features provide clues about the nodule’s nature and help guide decisions about what to do next.
Size
The size of a lung nodule is a primary consideration. Generally, smaller nodules are less likely to be cancerous. For example, nodules smaller than 5 millimeters have a very low probability of being malignant, often estimated at less than 1%. As the size increases, so does the level of suspicion. Nodules larger than 8 millimeters often warrant closer observation or further testing.
Shape and Borders
The shape and the nature of a nodule’s borders are also informative. Benign nodules often present with smooth, round, or regular borders. They may also have a flat, polygonal, or discoid shape. In contrast, malignant nodules frequently have irregular, spiculated (spiky), or lobulated (scalloped) edges. These spiky projections can suggest that the lesion is growing into the surrounding lung tissue.
Density and Calcification
The internal appearance, or density, of a nodule provides further clues. Solid nodules that contain calcium, especially in a central, laminated (layered), or “popcorn” pattern, are typically benign. These calcification patterns are often signs of old infections or benign growths like hamartomas. Conversely, nodules that are not solid, described as “ground-glass” or “part-solid,” can be more concerning. A part-solid nodule, which has both a ground-glass component and a solid part, may carry a higher suspicion, especially if the solid portion is larger than 8 millimeters.
Common Causes of Non-Cancerous Nodules
The majority of lung nodules are not cancerous and can arise from a variety of sources. Many of these benign formations are remnants of past healing processes or reactions to inflammation.
One of the most frequent causes of a benign nodule is a granuloma. A granuloma is a small, organized collection of inflammatory cells that the body forms to wall off substances it cannot eliminate. These are often the result of a past fungal infection, such as histoplasmosis, or a bacterial infection like tuberculosis. In many cases, the person was never aware they had the initial infection.
Certain inflammatory conditions can also lead to the formation of lung nodules. For individuals with autoimmune diseases like rheumatoid arthritis or sarcoidosis, nodules can develop in the lungs as part of the body’s systemic inflammatory response.
Finally, some nodules are benign tumors. The most common type is a hamartoma, which is a disorganized growth made of normal lung tissues like cartilage, fat, and muscle cells. Hamartomas often have a distinct appearance on CT scans, sometimes showing fat and “popcorn-like” calcification, which helps in their identification.
Follow-Up and Monitoring Procedures
After a lung nodule is detected, the next steps depend on its size, appearance, and the individual’s overall risk factors for lung cancer. For many small nodules that appear benign, the recommended approach is often “watchful waiting” or active surveillance. This involves a series of follow-up CT scans over a period of time to monitor for any changes.
The schedule for these follow-up scans is standardized, with imaging often repeated at intervals such as three, six, 12, and 24 months. This allows doctors to determine if the nodule is growing, as malignant nodules tend to grow. If the nodule remains stable over a two-year surveillance period, it is considered benign, and further scans may not be necessary.
If a nodule shows growth or has characteristics that are suspicious, further diagnostic tests may be recommended to obtain a more definitive answer. A positron emission tomography (PET) scan can help determine if the nodule’s cells are highly active, a potential sign of cancer. In cases where suspicion remains high, a biopsy may be performed to remove a small sample of the nodule tissue for examination under a microscope.