A pulmonary nodule is a small spot on the lung, typically found by accident during a CT scan done for another reason. It shows up as a rounded or irregular shadow, measuring 3 centimeters (about 1.2 inches) or less in diameter, surrounded by normal lung tissue. Roughly 30% of all chest CT scans reveal at least one pulmonary nodule, making them extremely common. The vast majority are not cancer.
How Common They Are and What Causes Them
Malignant nodules account for only 1 to 12% of all detected pulmonary nodules. The rest have benign explanations, and the single most common type is an infectious granuloma: a small cluster of immune cells that formed in response to a past infection. Fungal infections and tuberculosis are frequent culprits. Your immune system walled off the infection, and the granuloma is essentially a scar left behind. You may have had no symptoms at the time, or you may have had a mild respiratory illness you’ve long since forgotten.
Other causes include autoimmune conditions like rheumatoid arthritis and sarcoidosis, which can produce noninfectious granulomas. Inhaled environmental particles, old areas of lung scarring, and benign growths called hamartomas (small lumps of normal tissue that grew in an unusual spot) round out the list. Any opacity smaller than 3 millimeters is classified as a micronodule, and anything larger than 3 centimeters is no longer called a nodule but a lung mass, which carries a different risk profile.
Why Most People Have No Symptoms
Pulmonary nodules almost never cause symptoms on their own. They’re too small to block airways or press on surrounding structures. Most people learn they have one only because they had imaging done for something unrelated, like chest pain, a pre-surgical workup, or a car accident. This is why they’re often called “incidental findings.” If you do have a cough, shortness of breath, or chest pain, those symptoms are far more likely coming from whatever prompted the scan in the first place, not the nodule itself.
What Makes a Nodule More or Less Likely to Be Cancer
Size is the single strongest predictor. In a study of patients under 35, no malignant nodules were found among those measuring 10 millimeters or smaller. Larger nodules carry progressively higher risk. A nodule that has grown between two scans is more concerning than one that has stayed the same size over months or years.
Your personal risk factors also matter. A history of smoking, older age, a family history of lung cancer, or a prior cancer diagnosis all shift the probability upward. That said, in younger patients, even smoking history did not reach statistical significance as a predictor of malignancy, likely because cancer in small nodules is so rare in that age group. For people under 35, the malignancy rate for incidental nodules was just 0.3%.
Certain features on the CT scan itself offer clues. Nodules with dense calcification in specific patterns (popcorn-like, central, or layered) are almost always benign. Smooth, round borders also lean toward a harmless cause. By contrast, nodules with irregular or spiculated (spiky) edges, or those that show multiple scattered solid components inside a hazy area, raise more suspicion.
How Doctors Decide What to Do Next
Not every nodule needs further workup. The Fleischner Society, whose guidelines are widely used by radiologists, bases its recommendations on nodule size and your individual risk profile. For solid nodules smaller than 6 millimeters, the current guidelines recommend no routine follow-up imaging at all in most patients. The risk is simply too low to justify repeated scans.
For nodules in the 6 to 8 millimeter range, a follow-up CT scan is typically recommended somewhere between 6 and 12 months later. The goal is to check whether the nodule has changed size. Stability over time is reassuring. Surveillance can extend up to two years for some nodules, with repeat scans at intervals, before a nodule is considered stable enough to stop monitoring.
High-risk patients with nodules in the 7 to 8 millimeter range may be scanned sooner, sometimes within 3 to 6 months. If a nodule is 8 millimeters or larger and the calculated malignancy risk exceeds about 10%, a PET scan is often the next step. PET scans detect metabolic activity: cancer cells burn more energy than normal tissue and light up on the scan. In one prospective study, 20 patients avoided an invasive biopsy entirely because their PET scan showed little to no activity in the nodule, allowing doctors to recommend observation instead.
When a Biopsy Comes Into Play
Biopsy is generally reserved for nodules between 8 and 30 millimeters that have worrisome features, either on CT, PET, or both. A nodule that lights up brightly on a PET scan, has grown on serial imaging, or has irregular borders in a patient with risk factors is a candidate for tissue sampling. The biopsy can be done with a needle guided by CT imaging, through a bronchoscope (a thin tube passed through the airways), or occasionally through a small surgical procedure.
The decision is rarely made by one test alone. Most guidelines recommend discussing borderline cases in a multidisciplinary team that includes a radiologist, a pulmonologist, and a thoracic surgeon. The aim is to catch the small number of cancerous nodules early, when they’re most treatable, without subjecting the much larger number of people with harmless nodules to unnecessary procedures.
What to Expect if You Have One
If your report mentions a pulmonary nodule, the most likely outcome is that it’s benign and requires nothing more than a follow-up scan. Many people have multiple nodules, which actually tends to suggest a benign process like a past infection rather than cancer. The waiting period between scans can feel stressful, but the monitoring approach exists precisely because most nodules pose no threat, and jumping to biopsy for every small spot would cause more harm than good.
If follow-up imaging does show growth, the process moves quickly. Further imaging, a PET scan, and possibly a biopsy will clarify what’s happening. Early-stage lung cancers caught as small nodules have significantly better outcomes than cancers found at later stages, which is one reason lung cancer screening programs for high-risk individuals (long-term smokers over 50) use low-dose CT scans to look for exactly these kinds of findings.