When a chest X-ray or CT scan reveals a “spot on the lung,” the finding is medically termed a pulmonary nodule or lesion. These spots are abnormal growths or clusters of tissue distinct from the surrounding healthy lung tissue on an imaging scan. While this finding causes immediate concern, the vast majority of these incidentally found nodules are benign, meaning they are non-cancerous. The investigation aims to differentiate between potential causes, as many represent healed infections or inflammation that pose no threat to health.
What Does “Spot on the Lung” Actually Mean?
The medical term “pulmonary nodule” refers to a small, isolated spot in the lung, typically appearing as a white shadow on an X-ray or CT scan. By definition, a nodule is an area of abnormal growth measuring 3 centimeters (about 1.2 inches) or less in diameter. A lesion larger than 3 centimeters is classified as a pulmonary mass, which carries a higher likelihood of being malignant. Most lung nodules are discovered incidentally when a person undergoes a chest scan for an unrelated reason.
The characteristics of the spot provide initial clues to its cause. Features like calcification, where the tissue has hardened with calcium deposits, often suggest a long-standing, benign process, such as a healed infection. Radiologists examine the margins, or edges, of the spot; a smooth border is typically less concerning than an irregular or spiculated (spiky) one. Observing the size and shape over time is a key element in determining the spot’s true nature.
Causes Related to Past Infection and Inflammation
The most common reason for a lung spot is residual scarring left after the body successfully fought off a past infection or inflammation. These remnants often calcify over time, becoming stable and harmless. The resulting clusters of immune cells and scar tissue are known as granulomas, representing the immune system’s way of walling off an invading organism.
A frequent cause of infectious granulomas is exposure to Tuberculosis, even if the person never developed active disease. Fungal infections are another major contributor, particularly in certain geographic regions where specific fungal spores are common. Examples include Histoplasmosis in the Midwestern United States and Coccidioidomycosis (Valley Fever) in the Southwest. Healed bacterial pneumonia or abscesses can also result in persistent scar tissue visible as a nodule.
Systemic inflammatory conditions can also manifest as lung spots. Autoimmune diseases like Rheumatoid Arthritis can cause the development of rheumatoid nodules, which are non-infectious areas of inflammation. Sarcoidosis, characterized by the growth of inflammatory cells, frequently involves the lungs. These inflammatory nodules are generally not cancerous but do require monitoring.
When Lung Spots Indicate Cancer
While most nodules are benign, a lung spot can be the first sign of cancer, necessitating a thorough investigation. The risk of malignancy is strongly correlated with a person’s history, especially a significant history of smoking. Nodules that are new, growing, or have suspicious characteristics on the imaging scan raise the most concern for a cancerous process.
One possibility is primary lung cancer, which originates in the lung tissue itself. Suspicious features on a CT scan include nodules with irregular, lobulated, or spiculated margins, suggesting invasive growth into the surrounding tissue. A doubling time—the rate at which the nodule increases in volume—between 30 and 400 days is highly suggestive of malignancy. Nodules located in the upper lobes of the lung also tend to have a higher risk of being cancerous.
The lung spot might also represent metastatic cancer, meaning the cancer originated elsewhere in the body and spread to the lungs. Cancers of the breast, colon, kidney, or head and neck commonly metastasize to the lungs, often presenting as multiple nodules rather than a single spot. The presence of multiple lesions scattered throughout both lungs greatly increases the suspicion for a secondary cancer. Risk assessment integrates these imaging characteristics with a person’s age and overall health history.
How Doctors Investigate Lung Spots
Once a lung spot is identified, doctors use a methodical approach focusing on risk stratification and observation. For small nodules, typically those less than 6 millimeters, and in low-risk patients, the common strategy is a “wait and watch” approach. This involves active surveillance, where serial low-dose CT scans are performed every few months to check for any growth or change. A spot that remains stable for a minimum of two years is considered benign and usually requires no further follow-up.
If a nodule is larger, generally exceeding 8 millimeters, or displays features that raise concern, further diagnostic tests are often ordered. A Positron Emission Tomography (PET) scan assesses the metabolic activity of the spot. Cancer cells are typically more metabolically active than benign tissue, causing them to “light up” on the PET scan, which helps distinguish between benign and malignant lesions. PET scans are not reliable for very small nodules, usually those less than one centimeter.
The definitive diagnosis requires tissue sampling, known as a biopsy. This is reserved for high-risk nodules that are growing rapidly or have highly suspicious characteristics. A biopsy can be performed using a thin needle guided by a CT scan or through a bronchoscope, which is a flexible tube inserted through the airway. If the risk of cancer is very high, a surgeon may elect to remove the entire nodule surgically, serving as both the definitive diagnostic step and the potential treatment.