Low-dose CT scan

This page was reviewed under our medical and editorial policy by

Jason Beland, MD, Chair, Department of Radiology, City of Hope Atlanta, Chicago and Phoenix

This page was reviewed on April 25, 2022.

The prognosis for lung cancer patients is better when the disease is diagnosed early, which is why people at high risk of lung cancer are encouraged to undergo routine testing designed to detect whether cancerous growths are forming in their lungs.

Lung cancer screening with a low-dose computed tomography (CT) scan, also called an LDCT, of the chest—like a mammogram used to detect breast cancer or a colonoscopy performed to check for colon cancer—is a preventive tool that may catch signs of disease at an early stage, when it’s easier to treat.

This page will cover:  

What is a low-dose CT scan?

A low-dose CT scan is non-invasive and takes little time to perform. The screening test works just like a regular CT scan but with a significantly lower radiation dose, which decreases the risk associated with annual or more frequent examinations.

A 2020 Dutch-Belgian study, called the NELSON trial, analyzed the impact of LDCT screenings in more than 15,000 people over a 10-year period. Researchers reported that lung cancer deaths dropped about 33 percent in women and 24 percent in men when compared to those who did not receive the screenings. The American Cancer Society says widespread use of LDCT screenings could save up to 60,000 lives annually in the United States.

LDCT screening eligibility criteria

The U.S. Preventive Services Task Force recommends an annual LDCT lung screening for those who are:

  • Aged 50 to 80


  • A current or former smoker who quit within the past 15 years


  • A person with a heavy smoking history (defined as a 20 pack-year history, meaning one pack a day for 20 years, or two packs a day for 10 years, etc.)

If a person's job puts him or her at higher risk of lung cancer, he or she may also be advised to undergo an LDCT scan of the chest.

If the patient is a smoker younger than 50, a light smoker or quit more than 15 years ago, the test may not be recommended.

If the patient meets the recommended criteria, he or she is eligible for a lung cancer screening even if he or she continues smoking. However, the doctor will recommend the patient quit to lower his or her risk of not just lung cancer but more than a dozen other cancers. The patient's insurance provider may require him or her to talk to a health care provider about enrolling in a smoking-cessation program before the lung cancer screening.

What's the difference between a low-dose CT scan and a regular CT scan?

A regular CT scan works by producing a series of X-ray images or “slices” through a designated area of the body. It layers these cross-sectional images one upon another to create a detailed 3D picture of organs, bone or other tissues and to detect abnormalities.

CT imaging is frequently used in diagnosing, staging and treating cancer. The radiation emanating from a CT is similar to what the patient would typically be exposed to from the atmosphere and naturally occurring radioactive materials. Still, even small amounts of exposure may increase the risk of future cancers.

A low-dose CT scan has only one-fifth of the radiation of a regular CT scan, making it safer for routine testing. Technically, an LDCT scan provides about 1.4 millisieverts (mSv) of radiation (the scientific unit for measuring ionizing radiation), while a standard diagnostic CT scan is closer to 7 mSv, according to the Agency for Healthcare Research and Quality. The average background radiation a person may be exposed to in a year is 3 to 5 mSv.

The scan uses a lower dose of radiation because it is designed to evaluate nodules in low-density lung tissue but is less effective in evaluating bones, organs or other tissues.

Benefits of low-dose CT scans

The small radiation exposure from LDCTs allows for more frequent exams as part of a comprehensive lung cancer screening program. These routine exams may alert the care team to any new or changing nodules and may help determine whether further investigation is warranted.

The screenings may also identify potential tumors before the patient experiences any symptoms, which may not develop until the disease has progressed to an advanced stage. Early detection allows for earlier treatment and a greater likelihood of long-term survival. Lung cancer is the leading cause of cancer death in the United States.

Low-dose CT scan risks

An LDCT is not risk free, and the minor risk may not outweigh the benefits for someone at high risk for lung cancer. LDCT risks include:

Radiation exposure. Low-dose CT screening greatly reduces but doesn’t eliminate all radiation exposure from testing, so there’s always a risk a scan may contribute to the patient developing cancer. The amount of radiation from an LDCT is higher than from a traditional X-ray.

Added stress/anxiety. Testing may lead to increased anxiety and stress before the procedure and worry over possible results.

False positives or other abnormalities. A scan may produce a “false positive,” indicating an abnormality that is not cancer and may lead to further testing.

Overdiagnosis. Some low-grade cancers may not pose a significant health threat. However, once the disease is detected, the cancer care team may recommend treatments that ultimately were deemed unnecessary.

What to expect

If the patient meets the eligibility criteria for an LDCT, he or she should discuss the pros and cons of testing with the doctor.

Little preparation is needed for a low-dose CT scan. It doesn’t use needles or medications, nor does it require contrast material. Some tips include:

  • Consider wearing loose-fitting, comfortable clothing for the scan. Depending on where the exam is done, the patient may be asked to change into a gown.
  • Leave any metal objects, such as jewelry or body piercings, at home.
  • Tell the care team if the patient is pregnant or if a pregnancy is suspected.

When it’s time for the procedure, the patient lies flat on the scanner table. He or she may be strapped in and given pillows so he or she remains still and comfortable during the exam. Patients will be asked to raise their hands over their head for the chest scan.

A technologist may ask the patient to hold his or her breath for five to 10 seconds at a time. During the scan, the exam table passes through the CT scanner so that its X-ray beam can move around the patient  in a spiral path. The machine is donut-shaped and not a tunnel, so the patient shouldn’t have to worry about claustrophobia.

How long does a low-dose CT scan take?

The whole process takes only a few minutes, though the setup beforehand may add some time to the appointment. The radiologist sends an official report to the patient's doctor, including an interpretation of what the images showed and a recommendation for follow-up or further testing.

Understanding low-dose CT scan results

Positive results: If the scan is positive, it means an abnormality was detected. If a pulmonary nodule or other abnormality is found, the doctor will consult with the patient about what to do next. A positive scan doesn’t necessarily mean the patient has cancer, only that he or she may need follow-up or additional testing to reach a conclusive diagnosis.

Negative results: A negative test means the scan didn’t detect anything abnormal. The care team will recommend when the patient should repeat the scan based on his or her health and history.

Indeterminate results: The care team may also classify the test results as indeterminate, which means the patient should follow up or have additional imaging.

LDCT screening timetable

Former heavy smokers are urged to begin screening when they turn 50 and for 15 years after they quit. Current heavy smokers are advised to begin testing at age 50 and to continue annually until age 80, or 15 years after quitting.

No matter the patient's age or when he or she quits, the doctor may refer him or her for continued screenings or recommend other procedures if there’s a concern about changes found in recent screenings. While scans are usually performed annually, they may also be repeated more frequently based on previous results.

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Show references
  • National Cancer Institute. Cancer Stat Facts: Common Cancer Sites.
  • U.S. Centers for Disease Control and Prevention (2016, Nov. 10). Cancers linked to tobacco use make up 40% of all cancers diagnosed in the United States.
  • U.S. Preventive Services Task Force (2021, March 9). Lung Cancer: Screening.
  • Innovate Healthcare (2020, Jan. 30). NELSON Trial Confirms Overwhelming Benefits of CT Lung Cancer Screening.