Lung nodules, also known as pulmonary nodules, are small, abnormal spots or shadows that appear in the lung tissue on a chest X-ray or computed tomography (CT) scan. A nodule is typically measured at less than three centimeters in diameter; anything larger is classified as a mass. While finding a nodule can be alarming, these findings are common, and the vast majority—over 95% of small nodules—are not cancerous. Understanding the various reasons why a nodule forms provides context for the diagnostic steps that follow.
Benign Growths and Non-Infectious Scars
Many lung nodules are the result of past tissue damage or non-cancerous growths that pose no serious threat. A common benign cause is the hamartoma, the most frequent type of non-cancerous lung tumor. Hamartomas are composed of disorganized, normal tissues such as cartilage, connective tissue, and fat, and they typically appear as well-defined, round lesions.
Other nodules represent residual inflammation or scarring, a process known as fibrosis, from a prior non-infectious lung injury or minor trauma. Connective tissue disorders, where the immune system attacks healthy tissue, can also lead to nodule formation. For instance, individuals with rheumatoid arthritis can develop rheumatoid nodules in their lungs. These benign growths are usually stable in size and often only require monitoring rather than immediate treatment.
Infectious History and Granulomas
A significant number of lung nodules are granulomas, which are small, organized clumps of immune cells that form a wall around a foreign substance or infectious organism. The body creates these cellular capsules to contain an infection that the immune system has successfully fought off. These nodules represent a healed or inactive infection and are often found to be calcified, meaning they have hardened with calcium.
The most common infectious agents that result in granulomas are certain fungi and bacteria, even if the person never experienced severe symptoms. Fungal infections like Histoplasmosis or Coccidioidomycosis (Valley Fever) can leave behind these benign nodules. Tuberculosis, caused by the bacterium Mycobacterium tuberculosis, is another frequent cause of granuloma formation worldwide. The presence of these calcified granulomas indicates a historical encounter with the pathogen rather than a current, active disease.
Malignant Potential
While most nodules are benign, the possibility of malignancy—primary lung cancer or cancer that has spread from elsewhere—requires careful investigation. Primary lung cancer, such as non-small cell lung cancer, appears as a nodule in its early stages. A nodule is considered suspicious if it exhibits irregular or spiky borders, a feature known as spiculation, or if it is located in the upper lobes.
The risk of a nodule being cancerous rises proportionally with its size. Nodules smaller than six millimeters have a malignancy risk of less than 1%, but the risk increases to nearly 10% for nodules that are eight millimeters or larger. High-risk patient characteristics, such as a substantial smoking history, older age, or a personal history of cancer, also increase the probability that a nodule is malignant. Metastatic cancer, where malignant cells travel from a tumor in another organ, can also present as one or multiple pulmonary nodules.
Navigating the Diagnostic Process
When a lung nodule is discovered, the diagnostic process focuses on determining the probability of malignancy by assessing the nodule’s characteristics and the patient’s risk factors. The first step involves detailed imaging, typically a CT scan, to measure the nodule’s size, density, and shape. Characteristics like smooth edges and a solid, dense appearance suggest a benign cause.
The Fleischner Society Guidelines provide a standardized framework for the follow-up of incidentally detected solid nodules, differentiating management based on size and patient risk. For instance, single solid nodules smaller than six millimeters in a low-risk patient often require no routine follow-up. However, larger nodules or those in high-risk individuals necessitate repeat CT scans to monitor for growth, which is a strong indicator of malignancy.
If a nodule is eight millimeters or larger, or shows suspicious features, a Positron Emission Tomography (PET) scan may be used to assess its metabolic activity. Malignant nodules are often rapidly growing and metabolically active, causing them to “light up” on a PET scan. If the risk remains high after non-invasive tests, an invasive procedure like a needle biopsy or surgical removal may be necessary to obtain a tissue sample and confirm the nodule’s nature.