Pope Leo will undergo surgery in the coming days to remove a lung nodule that was detected during a routine check up. The 71-year-old said the growth was “well contained” and spotted early. So what exactly is a lung nodule, and what’s the link with cancer?Most are spotted incidentallyLung nodules are small growths found in the spongy tissue of the lungs. They are no more than 3 centimetres in diameter, and can be round or irregularly shaped. Anything larger is referred to as a mass.Lung nodules are usually detected by imaging of the chest, often using an X-ray or a computed tomography (CT) scan. While some people undergo screening specifically for lung nodules due to their increased risk of cancer, many nodules are picked up incidentally from unrelated scans. How common are lung nodules?Lung nodules are very common. In a study of 10,431 non-smoking (never or former smoking) participants given a chest CT scan in the northern Netherlands, 37.7% of women and 47.5% of men had at least one lung nodule. In the United States, lung nodules are detected in 31% of chest CT scans. The Australian National Lung Screening program suggests that lung nodules will be found in up to half of all people screened, but that most are neither symptomatic nor cancer-related. As imaging technology improves, and scans are performed more often, more lung nodules are likely to be detected. Read more: Australia’s lung cancer screening program is a year old. But can the health system deliver? What causes lung nodules?Older people, those who smoke, people with a family history of lung cancer and those exposed to asbestos are more likely to have lung nodules. Fortunately, most lung nodules are harmless. Around 95% of lung nodules tend to be benign (non-cancerous), and are instead caused by factors such as infections, immune-related diseases, and genetic abnormalities. However, in rare cases (3.7–5.5%), lung nodules may represent the early stages of lung cancer.When is a lung nodule dangerous?A major challenge for clinicians is deciding which nodules are likely to be harmless (or benign), and which are likely to be cancerous and require removal.Risk assessment considers factors such as the nodule size, density and location, as well as the patient’s age, smoking status and family history of cancer. It’s also important to refer to any previous medical imaging to detect whether nodule size or characteristics have changed. In some cases, a biopsy of the nodule may also be performed.Many tools are also being developed to help clinicians make these decisions, including machine learning, as a potential way to help determine whether a nodule is likely to be malignant. Biomarkers, such as proteins in the blood, or gene expression of surrounding lung cells, are also being investigated as ways to improve decision-making.If a nodule is causing pain or other symptoms, is growing rapidly, or is determined to be a high risk for cancer, it may be removed surgically. There are a few ways to remove the nodule.Open-lung surgery involves making an incision between the ribs to access the lungs. Other, less-invasive surgeries involve smaller chest incisions that allow a camera and surgical tools to access the lung. A surgeon can then cut out the nodule, either directly or with robotic assistance.What does it mean for cancer risk?If a nodule is confirmed to be cancerous after removal, then follow-up treatment may be recommended to ensure the cancer hasn’t spread. The risk of the cancer returning after the cancerous nodule is removed depends on the particular type of lung cancer diagnosed. One study on early-stage, non-small cell lung cancer showed the risk of cancer recurrence was 6–10% for the first four years after surgery, and then decreased to 2% per year. The risk of developing a new lung cancer remained at 3–6% and did not change over time. This is compared to a 0.055% chance of developing lung cancer per year in the general population (55 cases per 100,000 people per year).Lung cancer remains the most deadly form of cancer in Australia, as it is usually detected at a late stage, when treatment is very difficult. It is hoped that the new National Lung Cancer Screening program will help to detect lung cancer early in those at high risk. If you have concerns, speak to your doctor.Sarah Diepstraten receives funding from the National Health and Medical Research Council, Cure Cancer Australia and My Room Children's Cancer Charity. John (Eddie) La Marca receives funding from Cancer Council Victoria. He is affiliated with the Olivia Newton-John Cancer Research Institute and the Walter and Eliza Hall Institute of Medical Research.