A lung nodule is a small, abnormal spot on the lung, discovered incidentally during a chest X-ray or CT scan performed for other reasons. These findings are common, appearing in about one of every 500 chest X-rays. While the discovery can be concerning, the majority of these nodules are noncancerous (benign) and are often the result of a past infection or inflammation.
Distinguishing Features
Radiologists evaluate several characteristics on imaging scans to assess if a lung nodule is benign or malignant. One of the primary factors is size. Smaller nodules, especially those under 5 to 8 millimeters, are more likely to be benign. As a nodule’s size increases, so does the probability of malignancy, though about 55% of malignant nodules are still less than 2 centimeters in diameter.
The shape and border of a nodule provide clues. Benign nodules often have smooth, regular, and well-defined borders. Malignant nodules are more likely to have irregular, spiculated (spiky), or lobulated (scalloped) edges. These shapes can suggest the nodule is growing into surrounding lung tissue.
A nodule’s internal composition, specifically the presence of calcification, is another feature. Certain patterns of calcium deposits, which appear white on a CT scan, are strong indicators of a benign condition. Patterns like central, diffuse, or “pop-corn” calcification are associated with old infections or benign growths like hamartomas. While some cancerous nodules have calcification, it is less common and appears in a more punctate or amorphous pattern.
The stability of a nodule over time is a key indicator of its nature. A nodule that does not grow or change on follow-up imaging over two years is considered benign. Malignant nodules tend to grow, with a doubling time between 30 and 400 days. A very rapid doubling time of less than 30 days points towards an infection rather than cancer.
Causes and Risk Factors
Benign nodules are frequently the result of healed infections. Fungal infections like histoplasmosis or bacterial infections like tuberculosis can leave behind small, scarred areas that appear as nodules. Other causes include inflammatory conditions, such as rheumatoid arthritis, and noncancerous growths like hamartomas, which can contain fat, a reliable sign of a benign lesion.
The risk of a nodule being malignant is tied to an individual’s medical history and lifestyle. The most significant risk factor is a history of smoking. Other considerations include older age and a personal or family history of lung cancer. Occupational or environmental exposure to substances like asbestos or radon also increases this risk.
The Diagnostic Process
After a lung nodule is detected, the next steps depend on its size, features, and the patient’s risk profile. For small nodules that appear benign, the approach is often “watchful waiting” or active surveillance. This involves monitoring the nodule with a series of CT scans to check for growth or changes. If the nodule remains stable, it confirms its benign nature.
For nodules with more suspicious characteristics, further imaging may be recommended. A Positron Emission Tomography (PET) scan can assess a nodule’s metabolic activity. Cancer cells are more metabolically active and absorb more of a radioactive tracer used during the scan, causing them to “light up.” This helps differentiate between potentially malignant and benign nodules.
When imaging and risk factors suggest a high probability of malignancy, a biopsy is the definitive method for diagnosis. This procedure involves obtaining a small tissue sample from the nodule for examination by a pathologist. The sample can be collected through a thin tube passed down the airways (bronchoscopy) or with a needle guided by a CT scan.