Lung nodules: what a spot on a lung scan means, how often it is cancer and how doctors decide what to do

Pope Leo XIV told a crowd on October 11 that doctors had found a small nodule in one lung. Two days later, on Tuesday, October 13, the pope had surgery in Rome, and the Vatican announced the result. The Vatican said Leo was diagnosed with lung cancer Tuesday and had part of the right lung removed, NBC News reported.

More than a million Americans are told every year that a scan has shown a lung nodule, and for nearly all of them the story ends differently. A nodule is a description of what a scan shows. Most turn out to be harmless, and most are never operated on. This explainer sets out what a lung nodule is, how often one is found, how often it is cancer and how doctors choose between watching and acting.

What a lung nodule is

A lung nodule is a small, roughly round spot that looks denser than the lung around it on a scan. The Associated Press, in a report carried by ABC News, gave the usual size limit. A lung nodule is a small abnormal area measuring less than 3 centimeters (about 1.2 inches) across. Anything larger is called a mass.

Doctors use the word pulmonary, which means relating to the lungs, so a lung nodule and a pulmonary nodule are the same thing. Most are found on computed tomography (CT), a scan that builds a detailed picture of the chest from many X-ray images.

Nodules have many causes. Citing the American Cancer Society, NBC News listed irritants such as air pollution and diseases such as rheumatoid arthritis and cancer, and added that they can also be caused by scar tissue from previous infections.

A 2022 review in JAMA, a leading medical journal, sorts nodules by how they look on CT. Solid nodules are dense throughout. Subsolid nodules are hazier, and the review divides them into ground-glass nodules (no solid component) and part-solid (both ground-glass and solid components). Ground-glass means a faint, misty patch through which the normal lung can still be seen. The distinction matters because each type carries a different chance of cancer.

How common lung nodules are

Very common. The JAMA review says pulmonary nodules are detected on approximately 30% of computed tomographic (CT) images of the chest, and in about 1.6 million patients a year in the United States.

The yearly figure is close to an estimate from a study of a large US health system. Its authors counted every chest CT done for adult members over seven years. Between 2006 and 2012, more than 200,000 adult members underwent 415,581 chest CT examinations. Over that time the share of scans that showed a nodule rose from 24% to 31%.

Scaled up to the country, the numbers were large. The authors estimated that more than 4.8 million Americans underwent at least one chest CT scan and 1.57 million had a nodule identified, including 63,000 who received a new lung cancer diagnosis within 2 years. That is about 4 people in every 100 with a nodule.

Most of these nodules were incidental, which means found by chance on a scan done for some other reason. The study’s conclusion was that more scanning was finding more nodules and no more cancer. More frequent nodule identification has not been accompanied by increases in the diagnosis of cancerous nodules.

The pope’s nodule was found in a similar way. According to the Associated Press, the Vatican said Monday that no particular health issue prompted the checkup, which it described as routine.

How often a lung nodule is cancer

Benign means not cancer, and malignant means cancer. On the JAMA review’s figures, the great majority of nodules are benign. At least 95% of all pulmonary nodules identified are benign, most often granulomas or intrapulmonary lymph nodes. A granuloma is a small knot of scar-like tissue left behind by an old infection or inflammation. Intrapulmonary lymph nodes are small immune glands that sit inside the lung.

Size is the strongest single guide. The review states that smaller nodules are more likely to be benign, and gives figures in millimeters (mm). The probability of malignancy is less than 1% for all nodules smaller than 6 mm and 1% to 2% for nodules 6 mm to 8 mm. Six millimeters is about a quarter of an inch.

A Canadian study shows how this plays out among people having lung screening scans. Researchers tracked every nodule found on the first scan in two groups. In the first group, 1,871 people had 7,008 nodules, of which 102 were malignant. In the second, 1,090 people had 5,021 nodules and 42 were malignant. Counted by person instead of by nodule, the study put the rate of cancer at 5.5% of those with nodules in the first group and 3.7% in the second.

Hazy nodules follow different rules, and doctors first wait to see whether one lasts. The review says ground-glass nodules have a probability of malignancy of 10% to 50% when they persist beyond 3 months and are larger than 10 mm in diameter. That is a higher chance than for a solid nodule of the same size, but the cancers involved behave differently. A malignant nodule that is entirely ground glass in appearance is typically slow growing.

For part-solid nodules, the dense portion is what counts. Larger solid components are associated with a higher risk of malignancy.

What raises the chance that a lung nodule is cancer

Size and type are part of the picture. The Canadian team used its data to work out which features of the person and the nodule predicted cancer. Predictors of cancer in the model included older age, female sex, family history of lung cancer, emphysema, larger nodule size, location of the nodule in the upper lobe, part-solid nodule type, lower nodule count, and spiculation. Emphysema is smoking-related damage to the air sacs. A lobe is one of the five sections of the lungs. Spiculation means a spiky edge.

One item on that list runs against intuition. Having fewer nodules went with a higher chance of cancer.

The team’s conclusion was that such features can be combined into a calculator. Predictive tools based on patient and nodule characteristics can be used to accurately estimate the probability that lung nodules detected on baseline screening low-dose CT scans are malignant. Baseline means the first scan.

Smoking is the main cause of lung cancer, but a nodule in a person who has never smoked can still be malignant. About 20% of lung cancer cases are diagnosed in people who have never smoked or used tobacco, according to the American Cancer Society. Leo is not known to have ever been a smoker.

How doctors decide what to do with a lung nodule

The JAMA review lists four things that guide the decision. They are the probability that the nodule is malignant, safety of testing, the likelihood that additional testing will be informative, and patient preferences.

For the smallest nodules, the answer is often to do nothing more. An international group of chest specialists called the Fleischner Society revised its guidance for incidental nodules in 2017, and loosened it. The minimum threshold size for routine follow-up has been increased, the group wrote, so fewer tiny nodules now lead to repeat scans. Follow-up times were also made flexible. They are now given as a range rather than as a precise time period, to leave room for individual risk and preference.

For nodules in the middle, the usual step is to look again later. A cancer grows and a scar does not. According to the review, nodules that are 6 mm to 8 mm can be followed with a repeat chest CT in 6 to 12 months, with the timing depending on the person’s risk and how the nodule looks.

Solid nodules of 8 mm or more get closer attention, and the review lists the choices. Management options include surveillance imaging, defined as monitoring for nodule growth with chest CT imaging, positron emission tomography-CT imaging, nonsurgical biopsy with bronchoscopy or transthoracic needle biopsy, and surgical resection. Positron emission tomography (PET) is a scan that shows how actively tissue is using sugar, which cancers tend to do. A biopsy means taking a sample of tissue to examine under a microscope. Bronchoscopy means passing a thin tube down the airways, and transthoracic means through the chest wall. Resection means cutting the nodule out.

Biopsies are useful and imperfect. Current bronchoscopy and transthoracic needle biopsy methods yield a sensitivity of 70% to 90% for a diagnosis of lung cancer. Sensitivity is the share of real cancers that a test picks up, so 10% to 30% of cancers are missed by a single biopsy.

Sometimes doctors skip the biopsy. Ankit Bharat, the chief of thoracic surgery at Northwestern Medicine, described to NBC News what that choice can signal. “When doctors decide to proceed directly to surgery rather than first obtaining a biopsy, it can suggest a higher level of concern on the lung scan,” Bharat said. Even then, the scan alone does not settle it. Finding a nodule on a scan doesn’t confirm a cancer diagnosis, Bharat said.

In the pope’s case the operation gave the answer. Leo was diagnosed with adenocarcinoma. NBC News described it as a type of non-small cell lung cancer, which tends to grow more slowly than other kinds of lung cancer. Before the result was known, Stephen Liu, a lung cancer specialist at Georgetown’s cancer center, told the Associated Press what an early catch can mean. If the nodule is cancerous and removed at an early stage, the prognosis is “excellent,” and surgery alone may be sufficient treatment, Liu said. Prognosis means the expected outcome.

Surgery has costs of its own, and Bharat set them out before the operation, when it was not yet known how much tissue would be taken. If surgeons needed to remove an entire lobe, the pope’s lung capacity could decrease by as much as 25%, Bharat said. Smaller operations take less. The Vatican’s announcement said part of the right lung was removed. The JAMA review counts a person’s other illnesses, such as heart and lung disease, among the things that shape the choice for a larger nodule.

Lung nodules and lung cancer screening

Screening looks for lung cancer in people without symptoms, using low-dose CT. It works by finding nodules, so it finds a great many harmless ones.

The largest trial shows the scale. From August 2002 through April 2004, the trial enrolled 53,454 persons at high risk for lung cancer at 33 U.S. medical centers. Half had three yearly low-dose CT scans and half had chest X-rays. Across the three rounds, 24.2% of the CT screening tests came back positive, and nearly all of those were false alarms. A total of 96.4% of the positive screening results in the low-dose CT group and 94.5% in the radiography group were false positive results. A false positive is a worrying result in a person who turns out not to have cancer.

The scans still saved lives. There were 247 deaths from lung cancer per 100,000 person-years in the low-dose CT group and 309 deaths per 100,000 person-years in the radiography group, a reduction of 20%. Person-years add up the time each participant was followed.

A later Dutch and Belgian trial measured nodules by volume and re-scanned the uncertain ones before sending anyone to a specialist. It reported that 9.2% of the screened participants underwent at least one additional CT scan, and only 2.1% were referred for a suspicious nodule. Among the 13,195 men in the main analysis, lung cancer deaths ran at 2.50 per 1,000 person-years with screening and 3.30 without.

On that evidence the US Preventive Services Task Force (USPSTF), an independent panel that assesses screening, backs yearly scans for one group. Its statement applies to adults aged 50 to 80 years who have a 20 pack-year smoking history and currently smoke or have quit within the past 15 years. A pack-year is one pack of cigarettes a day for a year, so 20 pack-years could be a pack a day for 20 years. The panel’s reasoning rests on timing. Early-stage lung cancer has a better prognosis and is more amenable to treatment.

What the lung nodule numbers cannot show

One person’s nodule. The percentages are averages across thousands of people. A 1% chance does not say which nodule is the one.

People outside the studies. The cancer rates in the Canadian study and the trial results come from current and former smokers who were scanned on purpose. The health-system study’s estimate of 1.57 million people a year comes from scans done in routine care within one health system and extended to the country by calculation.

How the estimates were made. The JAMA paper is a narrative review, which means its authors summarized the research they judged relevant instead of pooling it by a fixed method. Its probabilities are ranges drawn from several studies.

Whether watching is always safe. Repeat scanning assumes that a short delay does little harm if a nodule proves to be cancer. That holds best for small and slow-growing nodules.

The pope’s outlook. The public reports give a diagnosis and an operation. They do not give a stage, and it is not clear whether the pope will undergo additional treatment.

Whether a nodule in a particular person calls for a repeat scan, a biopsy or an operation is assessed by their doctor.

A lung nodule is a common thing to be told about, and it seldom turns out to be cancer.