Lung cancer starts when abnormal cells grow out of control in the lung. They can invade nearby tissues and form tumors.

The cancer cells can spread, or metastasize, to the lymph nodes and other parts of the body.

Most lung cancer is caused by smoking.

Lung cancer screening is a way to find some lung cancers early, when a cure is more likely.

Screening is done with a CT scan. It uses X-rays, or radiation, to make detailed pictures of your body. Experts recommend that these scans be done in medical centers that focus on finding and treating lung cancer.

Lung screening is only recommended for people age 50 and older who are or were heavy smokers. That means people with a smoking history of at least 20 pack years. A pack year is a way to measure how much you have smoked.

To figure out your pack years, multiply how many packs a day (assuming 20 cigarettes per pack) you smoke by how many years you have smoked. For example:

• If you smoked 1 pack a day for 15 years, that's 1 times 15. So you have a smoking history of 15 pack years.
• If you smoked 2 packs a day for 10 years, that's 2 times 10. So you have a smoking history of 20 pack years.
• If you smoked 1 1/2 packs a day for 20 years, that's 1.5 times 20. So you have a smoking history of 30 pack years.

The U.S. Preventive Services Task Force (USPSTF) recommends annual lung cancer screening if:^{footnote 3}

• You are 50 to 80 years old.
• And you have a smoking history of at least 20 pack years.
• And you still smoke, or you quit within the last 15 years.
• And you are in good health overall. (Having a serious health problem might mean that you couldn't or wouldn't want to have treatment for lung cancer. The treatment could be too high-risk, and it might not help you live longer.)

The USPSTF also recommends that you no longer need lung cancer screening if you have not smoked for 15 years or if you have a serious health problem.

An abnormal (positive) CT scan can be the first warning sign of lung cancer. And it means you'll need more tests. These may include more CT scans and invasive testing like a lung biopsy.

In a biopsy, the doctor takes a sample of tissue from inside your lung so it can be looked at under a microscope. A biopsy is the only way to tell if you have lung cancer. If the biopsy finds cancer, you and your doctor will have to decide how or whether to treat it.

The scan can suggest a problem when there is not one. This is called a false-positive result. It's common to get an abnormal result from this test even when you don't have cancer. Several things can cause an abnormal result, including small growths (nodules) that would never cause any harm. But without further tests, your doctor can't tell whether an abnormal finding is cancer, a harmless nodule, or something else.

The scan may also fail to find cancer that is there. This is called a false-negative result.

Your scan may also be normal (negative).

A lung cancer screening test can't tell if you have lung cancer. The CT scan only shows whether there is something other than normal tissue in the lungs.

Don't smoke. Most lung cancers are caused by smoking. If you have already quit smoking, you've taken the best step you can to prevent lung cancer. And if you still smoke, the best way to lower your chance of getting or dying from lung cancer is to quit. Your doctor may recommend medicines that can help you quit.

Screening doesn't prevent lung cancer. It can only find some cancers early, when treatment may be more likely to work.

Screening has been shown to lower the risk of dying from cancer in older people who were heavy smokers.

The higher your risk (the more you smoked and the longer you smoked), the more likely it is that annual screening will prevent death from lung cancer.

False-positive results

CT screening for lung cancer isn't perfect. It can show an abnormal result when it turns out there was not any cancer. This is called a false-positive result. This means you may need more tests to make sure you don't have cancer. These tests can be harmful and cause a lot of worry.

Overdiagnosis

Some lung cancers grow so slowly that they will never cause a problem and don't need treatment. You might have this type of lung cancer, but a CT scan can't tell whether it's harmless. So you may get treatment—including surgery, radiation, or chemotherapy—even though you don't need it. This is called overdiagnosis, or overtreatment.

Radiation exposure

Lung cancer screening is done with a CT scan that uses a low dose of X-rays, or radiation, to make detailed pictures of your lungs. A CT scan exposes you to less radiation than most of us are exposed to each year from the natural radiation around us. But some people then need one or more follow-up scans, which use higher doses.

Each time you have the screening and any needed follow-up scans will add to your total radiation exposure.

Radiation from X-rays is known to cause cancer in a very small number of people. Still, for most people who are at high risk for lung cancer, the benefit of getting screened every year outweighs the risk of getting cancer from radiation.

The National Lung Screening Trial (NLST) studied older people with a history of heavy smoking. The tables below show how the results of the NLST study might affect a group of 1,000 people who are at high risk for lung cancer and who choose to have yearly screening with CT scans.^{footnote 1}

*Based on the best available evidence (evidence quality: high)

Benefits

The quality of the evidence about the benefits of lung cancer screening is moderate.

For heavy smokers who qualify for annual screening, having this regular test offers them a better chance of not dying of lung cancer.

Take two groups of 1,000 people. One group has annual screening for 3 years, and one group does not. About 6½ years later, about 18 people in the screening group and about 21 people in the non-screening group will have died of lung cancer. That means the group that did not have annual screening had 3 more lung cancer deaths than the screening group.

But this is just an average. The higher your cancer risk, the more likely it is that annual screening will prevent death from lung cancer. The more pack years in your smoking history, the higher your lung cancer risk.

Risks

The quality of the evidence about lung cancer screening risks is moderate.

False-positive result and follow-up. A CT scan will find nodules or other problems that aren't cancer. This is called a false-positive result. This could cause you to have other tests or treatments that it turns out you didn't need, and they could cause their own problems.

Take a group of 1,000 people who have annual lung cancer screening. About 365 of them will have at least one false-positive result. Of the 1,000 people tested, 25 will have an invasive procedure, such as a lung biopsy, because of a false-positive result. And 3 out of 1,000 will have a major complication because of that procedure. Major complications include collapsed lung, major bleeding, and, in rare cases, death.

Overdiagnosis. Screening may find cancers that might never be life-threatening. This is called overdiagnosis. It could cause you to have treatment you don't need. And that treatment could cause its own problems.

Take a group of 1,000 people who have annual lung cancer screening. Experts estimate that about 4 of them will get cancer treatment they didn't actually need.

Radiation-caused cancer. There is a very small chance that the extra radiation exposure from annual lung cancer screening could cause a fatal cancer.

Understanding the evidence

Some evidence is better than other evidence. Evidence comes from studies that look at how well treatments and tests work and how safe they are. For many reasons, some studies are more reliable than others. The better the evidence is—the higher its quality—the more we can trust it.

The information shown here is based on the best available evidence. The evidence is rated using four quality levels: high, moderate, borderline, and inconclusive.

Another thing to understand is that the evidence can't predict what's going to happen in your case. When evidence tells us that 2 out of 100 people who have a certain test or treatment will have a certain result and that 98 out of 100 will not, there's no way to know if you will be one of the 2 or one of the 98.

Your doctor may offer annual screening if:

• You are 50 to 80 years old.
• And you have a smoking history of at least 20 pack years.
• And you still smoke, or you quit within the last 15 years.
• And you are in good health overall. (Having a serious health problem might mean that you couldn't have treatment for lung cancer. The treatment could be too high-risk, and it might not help you live longer.)
• And you think the risks (false positives, invasive procedures, overdiagnosis) are worth the benefit of finding cancer early.