A computed tomography (CT) scan, also called a CAT scan, is an imaging test that is useful for detecting cancer and determining its stage (the extent of cancer in the body). Doctors will often repeat these tests to help determine if cancer treatment is working or to look for signs that cancer has come back.

A CT scan creates a three-dimensional picture of the inside of the body using a series of x-ray pictures that are taken from many different angles. A computer compiles these images into a detailed, cross-sectional view, which gives a doctor a thorough picture of the area scanned. Because of its many benefits, the use of CT scans has increased in the past few decades. But, concerns have been raised about the safety of CT scanning because it uses a form of radiation. Recent research suggests that the use of CT scans may slightly increase cancer risk in the U.S. population.

Doctors have long known that exposure to high doses of radiation can cause cancer, often years later. Most of the data come from survivors of the atomic bombs in Japan during World War II. However, the doses of radiation these individuals received were much higher than a person receives with repeated CT scans.

Many experts have looked at the evidence regarding the safety of CT scans. Most doctors agree that a CT scan should be performed only if it is medically necessary. A CT scan should not be used to screen for disease in a healthy person without symptoms. But, a person diagnosed with cancer or suspected of having cancer can safely receive a CT scan because the benefits always outweigh the risks.

Benefits and risks of CT

If your doctor wants you (or your child) to have a CT scan, know the benefits and risks of this test.

Benefits:

• A CT scan painlessly images your internal organs and other parts of your body that standard x-rays cannot examine. This avoids the need for almost all exploratory surgeries.
  
  
• CT saves lives. A CT scan helps detect tumor(s), guide a biopsy, determine the stage of cancer and whether cancer has spread, and monitor the effectiveness of cancer treatment.
  
  
• A CT scan is fast-less than one second per image. Thus, in children, it often can be done without the need for anesthesia to prevent them from moving during the test.

Risks:

• A CT scan uses much higher radiation doses than a standard x-ray examination or most other medical imaging tests. For instance, the radiation dose of a CT scan of the head is 25 times higher than that of a head x-ray.
  
  
• Children are at higher risk than adults from the radiation in a CT scan because their bodies are more sensitive to radiation. They also have a longer lifetime left to develop cancer.
  
  
• Generally, a CT scan is not recommended for pregnant women because of the potential risk to the fetus.

Cancer risk and CT

Here is what is known about whether CT scans can cause cancer:

• Modern CT scanners give radiation doses that are much lower than those that most atomic bomb survivors received.
  
  
• A CT scan most often targets just one part of the body. Atomic bomb survivors received radiation to the whole body at one time.
  
  
• The chance of absorbed x-rays from a CT scan causing cancer in a person is very small, according to the U.S. Food and Drug Administration.
  
  
• No instances of cancer in a person have ever been linked directly to CT scans.

The radiation dose from CT scans depends on the size of the body part being imaged, the number of scans in a study, and many other factors. The dose should be tailored to the size of the patient and the type of CT scan.

Questions to ask your doctor

Before you or your child has a CT scan, ask your doctor the following questions:

• Why do I need a CT scan? (Why does my child need a CT scan?)
  
  
• Are there risks of having a CT scan?
  
  
• What are the risks of not having the test?
  
  
• Is a CT scan the best diagnostic examination for me (my child)? Or are there others we can substitute?
  
  
• Is the radiation dose of the CT scan the lowest possible dose that produces diagnostic images?
  
  
• Is the radiation dose based on my (my child's) weight?
  
  
• Does the benefit of a CT scan outweigh its risks?
  
  
• Does this CT scan duplicate any previous tests?

Keep records of your tests

You can avoid unnecessary radiation exposure by avoiding unneeded or duplicate CT scans. Keep track of when you have CT scans and other imaging tests, where you had the tests, and why you had them. If your doctor wants you to have a CT scan, show him or her your records and make sure you still need the scan.

More Information

Expert Perspective from ASCO on the Link Between Cancer Risk and the Increased Frequency of CT Scans

Computed Tomography (CT) Scan-What to Expect

Additional Resources

Mayo Clinic: CT Scans: Are They Safe?

Radiology Info: Radiation Risk and CT (CAT) Scanning