The discovery of a pulmonary or lung nodule is a frequent occurrence, often found unexpectedly during a chest X-ray or CT scan performed for an unrelated reason. A lung nodule is a small, abnormal growth of tissue in the lung. The vast majority of these findings are not cancer; estimates suggest that over 95% of detected lung nodules are benign, representing harmless remnants of past infections or other cellular activity.
What Lung Nodules Are
A lung nodule is defined by its size, appearing on imaging as a round or oval shadow within the lung tissue. A growth is classified as a nodule only if its diameter is less than 30 millimeters (3 centimeters); anything larger is termed a lung mass and suggests a higher suspicion for malignancy. Nodules can be singular (solitary pulmonary nodule) or multiple, which may suggest a widespread inflammatory or infectious process.
Doctors classify nodules based on their appearance and density on a CT scan. A solid nodule appears dense, while a subsolid nodule is less dense and categorized into two types. A pure ground-glass nodule appears hazy, and a part-solid nodule contains both a ground-glass component and a denser, solid core. The presence of calcium, known as a calcified nodule, often indicates a long-healed, benign process.
Common Benign Causes
The most frequent causes of lung nodules are benign, typically involving scar tissue left behind after the body has fought off an infection or responded to chronic inflammation. These remnants are often called infectious granulomas—tight clusters of immune cells that have walled off a foreign invader. Fungal infections common in certain geographic areas, such as histoplasmosis or coccidioidomycosis (Valley Fever), or past bacterial infections like tuberculosis, are the primary culprits that leave behind these scars.
A different category of benign nodules arises from chronic inflammatory conditions or localized benign growths. Autoimmune diseases such as sarcoidosis or rheumatoid arthritis can form localized nodules that mimic other growths on imaging. Hamartomas represent the most common type of benign tumor in the lung, composed of a disorganized mix of normal lung tissues like cartilage, fat, and muscle. These growths are slow-growing and often contain characteristic features like fat or popcorn-like calcification that confirm their benign nature.
Malignant Causes and Risk Stratification
While most nodules are benign, a small percentage are malignant and represent either primary lung cancer or metastatic disease that has spread from a tumor elsewhere in the body. Primary lung cancers, such as adenocarcinoma, are frequently found as lung nodules, especially in the form of part-solid or ground-glass nodules. Malignant nodules that originate elsewhere commonly spread from primary tumors in the breast, colon, or kidney.
Risk stratification utilizes a patient’s history and the nodule’s imaging characteristics to estimate the probability of malignancy. Key patient factors that increase risk include a history of significant smoking, older age, a personal history of any cancer, and a family history of lung cancer.
Nodule features highly suggestive of malignancy include a size greater than 8 millimeters, a rapid growth rate over time, and an irregular or spiculated border that appears jagged or star-like. Conversely, smaller size, regular margins, and stability over two years strongly suggest a benign process. Part-solid nodules are associated with a higher risk of slow-growing cancer than purely solid or pure ground-glass nodules.
The Diagnostic Process and Next Steps
The first step after a nodule is identified is to compare the new image to any previous chest scans the patient may have had. If the nodule has remained unchanged in size and appearance for at least two years, it is considered benign and requires no further follow-up. If no prior imaging exists, the management plan is determined by the nodule’s size and appearance combined with the patient’s risk profile.
For small, low-risk nodules, the standard of care is active surveillance, involving repeat CT scans to monitor for change. Guidelines recommend follow-up scans at intervals such as six, twelve, and twenty-four months to confirm stability and avoid unnecessary invasive procedures. This approach is safe because most malignant nodules grow slowly and remain highly treatable if growth is detected early.
If a nodule is larger, high-risk, or shows growth during surveillance, further diagnostic steps are necessary. A Positron Emission Tomography (PET) scan may be used to assess the metabolic activity of the nodule, as cancerous cells tend to be more metabolically active.
For the highest-risk nodules, the definitive step is tissue sampling through a biopsy, which can be performed using a needle guided by CT or via a bronchoscope. A surgical procedure to remove the nodule may sometimes be the first step when the probability of cancer is very high.