A lung nodule is a small, dense spot that appears on a lung image, such as a chest X-ray or a computed tomography (CT) scan. These findings are common, present in up to half of all adults who receive a chest CT scan. While a spot on the lung can cause concern, the vast majority of lung nodules, more than 95%, are not cancerous and are considered benign.
What Exactly is a Lung Nodule?
A pulmonary nodule is defined as a rounded or irregular opacity in the lung measuring up to 3 centimeters (cm) in diameter. Nodules are small clumps of tissue that appear as a white spot or shadow on imaging. Any similar lesion larger than 3 cm is classified as a lung mass, which carries a higher suspicion of malignancy and requires a different clinical approach.
Most lung nodules do not cause symptoms and are discovered incidentally when a person undergoes imaging for an unrelated condition, such as a respiratory infection or trauma evaluation. They are also found during lung cancer screening programs using low-dose CT scans in high-risk individuals. Because CT technology offers greater detail than a standard X-ray, it frequently detects very small nodules that would otherwise go unnoticed.
Nodules are categorized based on their appearance on a CT scan, which offers clues about their nature. They may be solid, meaning they completely block structures, or subsolid. Subsolid nodules include hazy ground-glass opacities or part-solid nodules that contain both solid and hazy components. The density and shape of the nodule are factors physicians use to assess potential risk.
The Common Reasons Lung Nodules Form
The high prevalence of lung nodules is primarily due to them being residual evidence of the body’s past fight against infection or inflammation. These non-cancerous formations are most frequently infectious granulomas, which are small collections of immune cells that have walled off a foreign invader. Granulomas can form after exposure to common fungal infections, like histoplasmosis or coccidioidomycosis, or bacterial infections such as tuberculosis.
Scar tissue from a previous severe respiratory illness or an old injury can also present as a nodule on an imaging scan. Non-infectious inflammatory conditions may also lead to nodule formation. These include autoimmune disorders such as rheumatoid arthritis and sarcoidosis, where the immune system causes inflammation in the lung tissue.
Benign tumors, such as hamartomas, are another less common, non-cancerous cause of lung nodules. Since the vast majority of nodules are benign, the initial discovery of a lung spot is far more likely to be a historical mark than a new problem. This high rate of benign findings supports a cautious approach to management.
Assessing Nodule Risk and Follow-Up
When a nodule is detected, physicians use a risk stratification process combining the nodule’s characteristics with the patient’s individual risk factors. Factors that increase the suspicion of cancer include older age, a history of significant smoking, and a personal or family history of cancer. However, the physical appearance of the nodule on the scan is the most immediate factor guiding the next steps.
Nodule size is a primary determinant of risk. For solid nodules, those under 6 millimeters (mm) have a very low risk of malignancy, often less than 1%. Nodules larger than 8 mm carry a higher probability of being cancerous, sometimes up to 15.2%, and require a more aggressive management plan. Other suspicious features include an irregular or spiculated margin, where the edges appear jagged or spiky, or a location in the upper lobes of the lung.
For low-risk nodules, the most common approach is active surveillance using serial low-dose CT scans. This monitoring checks for growth, since a nodule that remains stable in size over a two-year period is considered almost certainly benign. For medium-sized nodules (6 to 8 mm) or those in high-risk patients, follow-up CT scans are recommended at shorter intervals, typically 6 to 12 months.
If a nodule is larger than 8 mm or has concerning features, additional testing is necessary. A Positron Emission Tomography (PET) scan may be ordered to assess the nodule’s metabolic activity. Cancerous cells typically consume more sugar and will “light up” on the scan. A PET scan can significantly influence the decision to proceed with a biopsy.
If the risk of malignancy is high, a tissue biopsy or surgical removal may be recommended for a definitive diagnosis. For nodules with an intermediate risk, a biopsy is performed to obtain a tissue sample to determine the exact cause. The goal of this structured follow-up is to find the small percentage of malignant nodules early, while safely avoiding unnecessary invasive procedures for the vast majority that are harmless.