How is CT used in cancer?
CT is used in cancer in many different ways:
- To detect abnormal growths
- To help diagnose the presence of a tumor
- To provide information about the stage of a cancer
- To determine exactly where to perform (i.e., guide) a biopsy procedure
- To guide certain local treatments, such as cryotherapy, radiofrequency ablation, and the implantation of radioactive seeds
- 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?
Studies have shown that CT can be effective in both colorectal cancer screening (including screening for largepolyps) and lung cancer screening.
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 (optical) colonoscopy, a thorough cleansing of the colon is performed before this test. During the examination, air or carbon dioxide gas is pumped into the colon to expand it for better viewing.
The National CT Colonography Trial, an NCI-sponsored clinical trial, found that the accuracy of CT colonography is similar to that of standard colonoscopy. CT colonography is less invasive than standard colonoscopy and has a lower risk of complications. However, if polyps or other abnormal growths are found on CT colonography, a standard colonoscopy is usually performed to remove them.
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 non-colorectal abnormalities may be found. Some of these “extra-colonic” findings will be serious, but many will not be, leading to unnecessary additional tests and surgeries.
The NCI-sponsored National Lung Screening Trial (NLST) showed that people aged 55 to 74 years with a history of heavy smoking are 20 percent 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).
Despite the effectiveness of low-dose helical CT for lung cancer screening in heavy smokers, the NLST identified risks as well as benefits. For example, people screened with low-dose helical CT had a higher overall rate of false-positive results (that is, findings that appeared to be abnormal even though no cancer was present), a higher rate of false-positive results that led to an invasive procedure (such as bronchoscopy orbiopsy), and a higher rate of serious complications from an invasive procedure than those screened with standard x-rays. NCI’s Patient and Physician Guide: National Lung Screening Trial provides more information on the benefits and harms.
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.
People who think that they have an increased risk of lung cancer and are interested in 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 be aware that, because the technique is fairly new, some insurance plans do not currently cover it.
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. 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.