An abnormal spot found on a lung scan, often called a lung mass or nodule, is a source of immediate concern. The majority of these findings are not cancerous. Most spots detected incidentally on chest imaging are benign, representing healed infections, inflammation, or structural irregularities. This article focuses exclusively on the non-cancerous causes of lung masses and how they are distinguished from malignant growths.
Understanding Lung Nodules and Masses
The initial step in evaluating an abnormal spot on the lung is defining its size. A pulmonary nodule is a lesion measuring less than 3 centimeters (cm) in diameter. Anything 3 cm or larger is categorized as a pulmonary mass. This size distinction is significant because lesions exceeding 3 cm have a higher likelihood of being malignant. However, both nodules and masses can be benign or malignant, though the vast majority of smaller nodules are benign.
Common Causes of Non-Cancerous Lung Masses
Non-cancerous lung masses and nodules arise from previous infections, chronic inflammation, and developmental irregularities within the lung tissue. Understanding their origin helps differentiate them from cancerous tumors.
Infectious and Inflammatory Remnants
The most frequent cause of a benign lung mass or nodule is a granuloma, an organized clump of immune cells and inflamed tissue. Granulomas are typically remnants of a healed infection, representing the body’s successful attempt to wall off an invading microorganism. Common culprits include fungal infections such as histoplasmosis and coccidioidomycosis (Valley fever), which are prevalent in specific geographic regions. Tuberculosis also commonly results in these scarred lesions. Once the infection is contained, the granuloma often calcifies, leaving a dense, non-growing spot visible on a CT scan.
Other inflammatory conditions can cause benign masses by triggering non-infectious inflammation in the lung tissue. Autoimmune disorders like rheumatoid arthritis and sarcoidosis can lead to the formation of distinct masses known as rheumatoid nodules or sarcoid nodules. These inflammatory lesions can mimic a cancerous mass on imaging, requiring careful evaluation of the patient’s medical history.
Structural and Developmental Abnormalities
Benign lung growths also arise from disorganized but normal tissue, such as pulmonary hamartomas. Hamartomas are overgrowths of mature components normally found in the lung, such as cartilage, fat, and connective tissue. These growths are often slow-growing. A highly specific imaging characteristic suggesting a hamartoma is the presence of fat within the lesion or a distinct “popcorn” pattern of calcification. Identifying these features on a CT scan provides confidence in a benign diagnosis without the need for further invasive procedures.
Vascular and Other Uncommon Causes
Less common causes of non-cancerous masses include certain vascular malformations. A pulmonary arteriovenous malformation (AVM) is an abnormal connection between an artery and a vein in the lung. Although not solid tissue, AVMs can appear as a mass-like shadow on imaging, composed primarily of blood vessels. Other rare, benign tumors, such as bronchial adenomas, fibromas, and lipomas, can also present as a lung mass. These lesions result from the abnormal growth of specific cell types but do not possess the ability to spread to distant sites.
Diagnostic Evaluation and Monitoring
Once a lung mass is detected, doctors use imaging characteristics and patient history to estimate the probability of it being cancerous. Several features seen on a Computed Tomography (CT) scan strongly suggest a benign nature. These include smooth, regular borders, which contrast with the irregular or “spiculated” edges often seen with malignancy.
The presence of specific patterns of calcification is one of the most reliable signs of benignity. Patterns highly characteristic of old granulomas or hamartomas include:
- Diffuse calcification.
- Central calcification.
- Laminated calcification.
- The aforementioned “popcorn” calcification.
If previous imaging is available, stability in size over a period of at least two years is also a strong indicator that the mass is benign.
For masses with a low probability of cancer, surveillance imaging, or “watchful waiting,” is often recommended. This involves repeating the CT scan at specified intervals (e.g., every three, six, or twelve months) to monitor for growth. A lack of growth over a two-year period effectively confirms the mass is benign. If imaging characteristics are uncertain, or if the patient has a high risk for lung cancer, further evaluation is needed. This may include a Positron Emission Tomography (PET) scan, which assesses the metabolic activity of the mass, or a biopsy to obtain a tissue sample for definitive diagnosis. Although some benign inflammatory masses show activity on a PET scan, the test helps guide the decision for an invasive procedure.