Lung nodules are a frequent finding on chest imaging, representing small, rounded spots in the lungs. While the discovery of an unexpected spot can be alarming, the great majority of these growths are benign and do not represent cancer. Understanding the common nature of these findings helps provide important context. The assessment of a lung nodule focuses on its characteristics and the patient’s overall risk profile to determine the appropriate management.
What Exactly Are Lung Nodules?
A lung nodule, also known as a pulmonary nodule, is defined by size as a small, isolated spot found in the lung tissue. This opacity appears as a dense shadow on imaging tests like chest X-rays or Computed Tomography (CT) scans. Any lesion measuring 3 centimeters (about 1.2 inches) or less in diameter is considered a nodule.
If a similar growth measures larger than 3 centimeters, it is classified as a lung mass, which carries a higher suspicion of malignancy. Nodules are often found incidentally—detected during a scan performed for an unrelated reason. They rarely cause noticeable symptoms, meaning their detection is frequently the first sign of their existence.
How Common Are Lung Nodules?
The prevalence of lung nodules is high, especially with the increased use of advanced imaging technology like CT scans. Studies show that an incidental lung nodule is present in a wide range, from about 8% up to 51% of all CT examinations that include the lungs. A systematic review found the overall prevalence to be about 30% among participants.
The frequency is also high in targeted screening programs for high-risk individuals, such as heavy smokers, where nodules are detected in approximately 27% of participants during initial low-dose CT screening. Even among non-smokers, one study found that 42% of participants had at least one solid lung nodule. Crucially, less than 5% of all incidentally detected lung nodules turn out to be malignant.
Why Do Lung Nodules Form?
Lung nodules form as a result of various processes, most of which are benign and related to the body’s natural defense and repair mechanisms. The most frequent benign cause is the formation of granulomas, which are small clumps of inflamed tissue. These granulomas often represent the healed remnants of past infections, such as fungal infections like histoplasmosis or bacterial infections like tuberculosis.
Other non-cancerous causes include localized inflammation, scar tissue from previous lung injury, and benign tumors like hamartomas. These benign nodules tend to remain stable in size over time. When a nodule is malignant, it is either an early-stage primary lung cancer or a metastasis from another part of the body. Malignant nodules typically display rapid growth, often doubling in size over a period of months.
Monitoring and Next Steps After Detection
The management of a newly discovered lung nodule is guided by a risk stratification process that considers the patient’s health history and the nodule’s characteristics. Factors like age, smoking history, and a history of previous cancer are weighed against the nodule’s size, shape, and density on the CT scan. Nodules that are small, generally less than 6 millimeters, and found in low-risk patients have a malignancy risk of less than 1%, and often require no further follow-up.
For larger nodules or those with suspicious features like irregular or “spiculated” borders, a surveillance strategy is adopted. This involves follow-up low-dose CT scans performed at specific intervals, such as 6, 12, or 24 months, depending on the risk level. The goal of surveillance is to monitor for any change, particularly growth, which is the most reliable indicator of malignancy. If a nodule shows significant growth or presents with highly concerning features, further action, such as a Positron Emission Tomography (PET) scan or a biopsy, may be necessary.