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Computed Tomography (CT) Scans and Cancer

What is computed tomography?

Computed tomography (CT) is an imaging procedure that uses special x-ray equipment to create detailed pictures, or scans, of areas inside the body. It is sometimes called computerized tomography or computerized axial tomography (CAT).


Modern CT machines take continuous pictures in a helical (or spiral) fashion rather than taking a series of pictures of individual slices of the body, as the original CT machines did. Helical CT (also called spiral CT) has several advantages over older CT techniques: it is faster, produces better quality 3-D pictures of areas inside the body, and may detect small abnormalities better.

In addition to its use in cancer, CT is widely used to help diagnose circulatory (blood) system diseases and conditions, such as coronary artery disease (atherosclerosis), blood vessel aneurysms, and blood clots; spinal conditions; kidney and bladder stones; abscesses; inflammatory diseases, such as ulcerative colitis and sinusitis; and injuries to the head, skeletal system, and internal organs. CT imaging is also used to detect abnormal brain function or deposits in adult patients with cognitive impairment who are being evaluated for Alzheimer’s disease and other causes of cognitive decline.

What can a person expect during a CT procedure?

During a CT procedure, the person lies very still on a table, and the table passes slowly through the center of a large donut-shaped x-ray machine. With some types of CT scanners, the table stays still and the machine moves around the person. The person might hear whirring sounds during the procedure. At times during a CT procedure, the person may be asked to hold their breath to prevent blurring of the images.

Sometimes, CT involves the use of a contrast (imaging) agent, or dye. The dye may be given by mouth, injected into a vein, given by enema, or given in all three ways before the procedure. The contrast dye highlights specific areas inside the body, resulting in clearer pictures. Iodine and barium are two dyes commonly used in CT.

In very rare cases, the contrast agents used in CT can cause allergic reactions. Some people experience mild itching or hives (small bumps on the skin). Symptoms of a more serious allergic reaction include shortness of breath and swelling of the throat or other parts of the body. People should tell the technologist immediately if they experience any of these symptoms, so they can be treated promptly. Very rarely, the contrast agents used in CT can cause kidney problems for certain patients, such as those with impaired kidney function. Patient kidney function can be checked with a simple blood test before the contrast agent is injected.



Women should let their health care provider and the technologist know if there is any possibility that they are pregnant. Depending on the part of the body to be scanned, the provider may reduce the radiation dose or use an alternative imaging method. However, the level of radiation exposure in a CT scan is believed to be too low to harm a growing fetus.


How is CT used in cancer?

CT is used in cancer in many different ways:

  • To screen for cancer
  • To help diagnose the presence of a tumor
  • To provide information about the stage of a cancer
  • To help plan external-beam radiation therapy or surgery
  • To determine whether a cancer is responding to treatment
  • To detect recurrence of a tumor

How is CT used in cancer screening?


Colorectal cancer

CT colonography (also known as virtual colonoscopy) can be used to screen for both large colorectal polyps and colorectal tumors. CT colonography uses the same dose of radiation that is used in standard CT of the abdomen and pelvis, which is about 10 millisieverts (mSv) (1). (By comparison, the estimated average annual dose received from natural sources of radiation is about 3 mSv.) As with standard colonoscopy, a thorough cleansing of the colon is performed before this test. During the examination, air or carbon dioxide is pumped into the colon to expand it for better viewing.


Whether CT colonography can help reduce the death rate from colorectal cancer is not yet known, and most insurance companies (and Medicare) do not currently reimburse the costs of this procedure. Also, because CT colonography can produce images of organs and tissues outside the colon, it is possible that noncolorectal abnormalities may be found. Some of these "extracolonic" findings will be serious, but many will not be, leading to unnecessary additional tests and surgeries.

Lung cancer

The NCI-sponsored National Lung Screening Trial (NLST) showed that people aged 55 to 74 years with a history of heavy smoking are 20% less likely to die from lung cancer if they are screened with low-dose helical CT than if they are screened with standard chest x-rays. (Previous studies had shown that screening with standard chest x-rays does not reduce the death rate from lung cancer.) The estimated amount of radiation in a low-dose helical CT procedure is 1.5 mSv (1). Those who have never smoked tobacco products are considered to be at too low a risk of lung cancer to benefit from lung cancer screening.


The benefits of helical CT in screening for lung cancer may vary, depending on how similar someone is to the people who participated in the NLST. The benefits may also be greater for those with a higher lung cancer risk, and the harms may be more pronounced for those who have more medical problems (like heart or other lung disease), which could increase problems arising from biopsies and other surgery. However, because low-dose lung CT can produce images of organs and tissues outside the lung, it is possible that nonpulmonary abnormalities such as renal or thyroid masses may be found. As with extracolonic findings from CT colonography, some such findings will be serious, but many will not.

Under the Affordable Care Act, all Marketplace health plans, and many other health plans, must cover lung cancer screening with low-dose CT as a preventive care benefit for adults ages 55 to 80 years who have a 30 pack-year smoking history (e.g., smoked one pack of cigarettes per day for 30 years or two packs of cigarettes per day for 15 years) and are either a current smoker or a former smoker who quit within the last 15 years. People who think they might qualify for screening with low-dose helical CT should discuss the appropriateness and the benefits and risks of lung cancer screening with their doctors. They should also check with their health insurance company to determine whether this screening is covered by their insurance plan.

Medicare covers the costs of annual lung cancer screening with low-dose CT in beneficiaries considered to be at increased risk based on their smoking history. Annual screening is covered for Medicare beneficiaries who are between 55 and 77 years old; have no signs or symptoms of lung cancer; are either a current smoker or have quit smoking within the last 15 years; have smoked an average of one pack a day for 30 years (i.e., 30 pack years); and receive the screening and low-dose CT scan at a Medicare-approved radiology facility. Medicare coverage includes a prescreening counseling visit with the health professional who wrote the order to review the benefits and risks of lung cancer screening.

What is total, or whole-body, CT?

Total, or whole-body, CT creates pictures of nearly every area of the body—from the chin to below the hips. This procedure, which is used routinely in patients who already have cancer, can also be used in people who do not have any symptoms of disease. However, whole-body CT has not been shown to be an effective screening method for healthy people. Most abnormal findings from this procedure do not indicate a serious health problem, but the tests that must be done to follow up and rule out a problem can be expensive, inconvenient, and uncomfortable, and they may expose the patient to extra risks, such as from an invasive procedure like a biopsy that may be needed to evaluate the findings. In addition, whole-body CT can expose people to relatively large amounts of ionizing radiation—about 12 mSv, or four times the estimated average annual dose received from natural sources of radiation. Most doctors recommend against whole-body CT for people without any signs or symptoms of disease.

What is combined PET/CT?

Combined PET/CT uses two imaging methods, CT and positron emission tomography (PET), in one procedure. CT is done first to create anatomic pictures of the organs and structures in the body, and then PET is done to create pictures that provide functional data about the metabolic pathways (chemical reactions that take place in a cell to create and use energy) that are active in tissues or cells. Cancer cells often use different metabolic pathways than normal cells.

Patients undergoing a PET/CT procedure are administered a positron-emitting (radioactive) substance, or radiopharmaceutical, that is designed to target cancer cells specifically. Numerous radiopharmaceuticals have been developed. However, the most common PET procedure uses an imaging agent called FDG (a radioactive form of the sugar glucose). Because cancerous tumors usually metabolize glucose more rapidly than normal tissues, they take up more FDG and appear different from other tissues on a PET scan. 

Other PET imaging agents can provide information about the level of oxygen in a particular tissue, the formation of new blood vessels, the presence of bone growth, whether tumor cells are actively dividing and growing, and whether cancer may have spread.


Is the radiation from CT harmful?



The widespread use of CT and other procedures that use ionizing radiation to create images of the body has raised concerns that even small increases in cancer risk could lead to large numbers of future cancers (2, 3). People who have CT procedures as children may be at higher risk because children are more sensitive to radiation and have a longer life expectancy than adults. Women are at a somewhat higher risk than men of developing cancer after receiving the same radiation exposures at the same ages (4).


What are the risks of CT scans for children? 


Individuals who have had multiple CT scans before the age of 15 were found to have an increased risk of developing leukemia, brain tumors (6), and other cancers (7) in the decade following their first scan. However, the lifetime risk of cancer from a single CT scan was small—about one case of cancer for every 10,000 scans performed on children. 




  • The Image Gently campaign was launched in 2008 to raise awareness about methods to reduce radiation dose during pediatric medical imaging examinations. The campaign is an initiative of the Alliance for Radiation Safety in Pediatric Imaging, a coalition founded by the Society for Pediatric Radiology, the American Society of Radiologic Technologists, the American College of Radiology, and the American Association of Physicists in Medicine. The Image Gently website provides information about pediatric imaging for radiologic technologists, medical physicists, radiologists, pediatricians, and parents.

  • In 2009, the American College of Radiology, the Radiological Society of North America, the American Society of Radiological Technologists, and the American Association of Physicists in Medicine created the Image Wisely campaign with the objective of lowering the amount of radiation used in medically necessary imaging studies and eliminating unnecessary procedures. The Image Wisely website provides resources and information to radiologists, medical physicists, other imaging practitioners, and patients.


  • The NIH Clinical Center requires that radiation dose exposures from CT and other imaging procedures be included in the electronic medical records of patients treated at the center (8). In addition, all imaging equipment purchased by NIH must provide data on exposure in a form that can be tracked and reported electronically. This patient protection policy is being adopted by other hospitals and imaging facilities.

What is NCI doing to improve CT imaging?

Researchers funded by NCI are studying ways to improve the use of CT in cancer screening, diagnosis, and treatment. NCI also conducts and sponsors clinical trials that are testing ways to improve CT or new uses of CT imaging technology. Some of these clinical trials are run by the ECOG-ACRIN Cancer Research Group, one of five groups in NCI’s National Clinical Trials Network. The American College of Radiology Imaging Network, which is now part of ECOG-ACRIN, performed the National CT Colonography Trial, which tested the use of CT for colorectal cancer screening, and participated in the NLST, which tested the use of CT for lung cancer screening in high-risk individuals

NCI’s Cancer Imaging Program (CIP), part of the Division of Cancer Treatment and Diagnosis (DCTD), funds cancer-related basic, translational, and clinical research in imaging sciences and technology. CIP supports the development of novel imaging agents for CT and other types of imaging procedures to help doctors better locate cancer cells in the body. In addition, CIP maintains the Cancer Imaging Archive, which is a library of medical images of cancer, including low-dose CT images, that are accessible for public download. This library has been used extensively by outside researchers in developing computer-aided diagnosis to help radiologists interpret CT images, for example, in lung cancer screening. 

Where can people get more information about CT?



Selected References


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