Intrapulmonary lymph nodes (IPLNs) are normal, small components of the immune system found deep within the lung tissue. While the discovery of a nodule on a lung scan can cause anxiety, finding an IPLN is a very common occurrence with modern, high-resolution imaging. These structures are part of the body’s defense mechanism, and their visibility does not typically represent a health threat. The vast majority of IPLNs are benign structures that require routine monitoring, but they must be differentiated from potentially harmful nodules.
What Intrapulmonary Lymph Nodes Are
Intrapulmonary lymph nodes are small, organized clusters of immune cells located within the lung parenchyma. They form part of the body’s extensive lymphatic system, acting as filters to trap foreign materials and fight infection. Their primary function is to collect and filter lymph fluid, removing inhaled foreign particles, cellular debris, and pathogens that reach the deep lung tissue.
These nodes are distinct from the larger lymph nodes found in the central chest, such as the hilar or mediastinal nodes. IPLNs are often situated near the pleura (the membrane lining the lungs) or along the interlobular septa (connective tissue walls that divide the lung segments). Their deep location makes them unique filters that constantly process material from the air we breathe. Continuous exposure to airborne irritants and dust explains why these nodes often become visible on medical imaging.
Why They Appear on Lung Scans
People learn about their IPLNs due to significant advancements in diagnostic technology, particularly computed tomography (CT) scans. Modern CT scanners produce thin-section images that reveal structures as small as a few millimeters, which were previously invisible on standard X-rays. This enhanced detection capability means these small, normal anatomical structures are now frequently seen as incidental findings.
IPLNs typically appear on a CT scan as solid, well-defined nodules, usually measuring less than 12 millimeters in diameter. They often have a characteristic ovoid, triangular, or polygonal shape, distinguishing them from the spherical appearance of many malignant tumors. Their visibility results from accumulating harmless substances, such as carbon particles or dust, which increases their density against the air-filled lung tissue. Due to their common location along the lung fissures, these nodes are frequently referred to as perifissural nodules (PFNs).
Indicators of Potential Danger
The primary concern is differentiating the IPLN from a cancerous nodule, and specific imaging features help radiologists make this distinction. Size is a primary criterion; IPLNs are generally considered suspicious when they exceed 12 millimeters in diameter, though some guidelines use a 10-millimeter threshold. Nodes smaller than 10 millimeters with typical benign features are rarely malignant.
Morphology and Growth
Beyond size, the node’s morphology—its shape and borders—is a strong indicator of its nature. A typical, benign IPLN is often oval, lenticular, or triangular, with sharply defined, smooth margins, and is frequently attached to a pleural surface or fissure. In contrast, a potentially dangerous nodule is more likely to be round, have irregular or spiculated (spiky) borders, and may show rapid growth. While some benign IPLNs can grow due to inflammation, abnormal growth, such as a 50% increase in volume, is a warning sign.
Location and Metabolic Activity
Other features like location and metabolic activity also influence the assessment. Benign IPLNs are more common in the middle or lower lobes, while those in the upper lobes can warrant extra attention. Positron emission tomography (PET) scans may assess the metabolic activity of larger nodes (typically exceeding 8 to 10 millimeters). A high uptake of the tracer can suggest malignancy or an active inflammatory process like tuberculosis or a fungal infection.
Management and Monitoring
When a typical intrapulmonary lymph node is found, the standard approach is conservative surveillance rather than immediate intervention. For small IPLNs, particularly those less than 6 millimeters with classic benign characteristics, many guidelines recommend no further follow-up is necessary. The risk of malignancy in these small lesions is extremely low.
For nodes that are slightly larger or have features that are not perfectly classic for a benign IPLN, serial imaging surveillance is usually recommended. This involves a repeat low-dose CT scan, often scheduled at 6 or 12 months for the first one to two years, to confirm stability. If the node remains stable in size and appearance over this period, it is considered benign, and follow-up can be discontinued.
If an intrapulmonary nodule is large, demonstrates suspicious morphological features, or shows significant growth, a more aggressive diagnostic procedure is required. This may involve a tissue sample collection, such as a transbronchial needle aspiration or a surgical biopsy, to determine the exact cellular composition. The goal of this structured monitoring is to safely distinguish harmless IPLNs from the rare nodules that require intervention, preventing unnecessary procedures while ensuring early detection.