Low-dose CT (LDCT) scan

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science at CTCA.

This page was updated on May 6, 2022.

A computed tomography (CT) scan is helpful in screening for, diagnosing, staging and treating cancer. A CT is a type of X-ray that creates detailed image “slices” of organs, bones and other tissues. When compiled on a computer, these slices provide a high-quality, 3D picture of areas inside the body and allow your care team to detect small abnormalities.

As a form of X-ray, CT involves ionizing radiation. Although the amount of radiation from a CT scan isn’t much higher than radiation that comes from space and naturally occurring radioactive materials such as radon, even small amounts of exposure may increase the risk of future cancers.

As a result, when ordering a CT scan, your doctor may request that technicians use the lowest dose possible to achieve the necessary results. Sometimes, too, your doctor may specifically order a low-dose CT (LDCT), which—according to the U.S. Food and Drug Administration—may be adjusted to as much as five times lower the amount of radiation than what the body absorbs from a full-dose or regular CT scan.

According to the Agency for Healthcare Research and Quality:

  • An LDCT scan provides 1.4 millisieverts (mSv) of radiation (the scientific unit for measuring ionizing radiation).
  • A regular diagnostic CT scan provides 7 mSv.
  • The average background radiation a person may be exposed to in a year in the United States is 3 to 5 mSv.

Low-dose CT scan for lung cancer screening

An LDCT scan is the recommended screening test for lung cancer. The U.S. Preventive Services Task Force recommends an LDCT lung cancer screening every year if you:

  • Are a current smoker or have quit within the past 15 years
  • Have a history of heavy smoking (defined as a 20 pack-year history; that’s one pack a day for 20 years or two packs for 10 years, etc.)
  • Are between age 50 and 80

If you’re a light smoker or quit more than 15 years ago, the test may not be particularly helpful.

If a job puts you at elevated risk of lung cancer, you may be advised to undergo an LDCT scan of the chest.

How do I prepare for my test?

Because an LDCT scan doesn’t require any contrast, you may not have to do anything special to prepare.

  • Consider wearing loose-fitting, comfortable clothing for the scan. Depending on where you have your exam done, you may be asked to change into a gown.
  • Leave any metal objects, such as jewelry or body piercings, at home.
  • Inform your care team if you’re pregnant or suspect you could be.

According to the Radiological Society of North America (RSNA) and the American College of Radiology (ACR), the table weight limit for a scan is typically 450 pounds. Ask your care team about alternatives if this applies to you.

How a low dose-CT scan is performed

When it’s time for the procedure, you may be asked to lie flat on the scanner table. You may be strapped in and given pillows so that you remain still during the exam. A technician may ask that you hold your breath for five to 10 seconds at a time.

For a lung LDCT, you may be asked to raise your hands over your head.

During the scan, the exam table passes through the scanner so that its X-ray beam can move around you in a spiral path.

The scan itself should take only a few minutes. Setup beforehand may add some time to the appointment.

A computer records the layered images and puts them together to create 3D images for the radiologist to review. The radiologist may send an official report to your doctor and include an interpretation of what the images showed.

Benefits of low-dose CT screening

An LDCT scan provides quality images that help detect small abnormalities using 90 percent less ionizing radiation than a standard CT scan of the chest, according to the RSNA and the ACR.

Moreover, this type of scan is painless and noninvasive, and ionizing radiation doesn’t remain in the body afterward. Lung cancer screening with LDCT also doesn’t require contrast material.

Risks of low-dose CT

Beyond the risk of radiation exposure from the initial LDCT scan, if an abnormality is found, your care team may recommend additional imaging tests be performed—exposing you to further radiation risks.

Sometimes the cancer detected is small and may not cause problems. This is known as overdiagnosis and may lead to your care team recommending unnecessary treatments.

Sometimes the results will come back positive (showing cancer), even though there is no cancer. This is a false-positive scenario, and it may lead to follow-up and treatments that aren’t necessary.

What do the test results mean?

If the scan is positive, it means something abnormal was detected. If a nodule or abnormality is found, more testing is needed to determine next steps.

A negative test means nothing abnormal was seen on the scan. Ask your care team when and if you should repeat the scan based on your health and history.

Your care team may also classify the test results as indeterminate, which means the scan is neither positive nor negative, and you should follow up with more scans at a later time.

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