​Computed tomography (CT) scan for cancer

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on March 2, 2022.

Your doctor may order a computed tomography (CT) scan, which is also known as a computed axial tomography (CAT) scan and a spiral or helical CT. The different names all mean the same thing.

  • A CT scan is an X-ray study that uses a computer to produce 3D cross-sectional images of the body.
  • The CT scanner obtains detailed pictures of internal organs, bones and soft tissues—and differs from a medical X-ray image in that it uses a doughnut-shaped machine to emit X-ray beams at different angles.
  • Some CT scans require a contrast or dye to be injected to get clearer images. In this case, your doctor will order a CT with contrast.

Can a CT scan detect cancer?

A CT scan, like any imaging tool, cannot detect cancer, though it may be useful in helping to identify a mass and determine its location and size. A CT scan may also offer valuable information, such as its shape and possible makeup (e.g., solid vs. liquid), that suggests the mass may be cancerous, but only a pathology review of tissue under a microscope following a biopsy can definitely determine a cancer diagnosis.

What does a CT scan show?

A CT scan can show whether you have a tumor—and, if you do, where it’s located and how big it is. CT scans can also show the blood vessels that are feeding the tumor. Your care team may use these images to see whether the cancer has spread to other parts of your body, such as the lungs or liver. The images are black and white.

It’s important to note that some cancers may be overlooked on a CT scan. Lesions may be missed for a variety of reasons, including location and human error. Still, CT is more sensitive than a simple X-ray.

A CT scan can find lesions as small as 2-3 mm. However, the location of the tumor may play a role in how big it must grow before it’s visible.

Compared to traditional X-rays, CT scans can provide more information about the size of suspicious nodules and how harmful they may be. They can be especially helpful when performed with an injection of material called contrast. Contrast is used to make certain tissues more visible. Cancer cells take up the contrast, which makes them appear white on the scan. This in turn allows your radiologist to better interpret the images, which is important when making a diagnosis. He or she will also be able to more clearly see tissues surrounding a potentially cancerous lesion, including nearby organs.

A CT scan with contrast may also be used to help determine treatment. For instance, using contrast can help tell whether the cancer can be removed with surgery.

Types of CT scans


3D CT pinpoints the location of tumors and can determine whether the cancer has spread to other parts of the body, and assesses the effects of treatment.

Advantages may include:

3D CT angiography

CT angiography (CTA) highlights the blood vessels of tumors and notes other abnormal blood vessels that may be a health concern.

Advantages may include:

  • Less invasive evaluation of blood vessels than catheter angiography
  • Capturing 3D images of almost every blood vessel in the body, as well as the brain, heart, lungs, pelvis, abdomen and extremities
  • The scan itself is quick, about 20 minutes, though the procedure may take a few hours from start to finish.

Large bore CT scanner/radiation therapy (RT) with simulation

This health-care tool produces detailed images of areas inside the body for planning radiation therapy. It uses simulation, fluoroscopy and respiratory gating to plan and deliver radiation.

Advantages may include:

  • Locating abnormalities and guiding precision radiation therapy
  • Planning for treatment options based on a patient’s breathing patterns
  • Allowing for varying patient sizes and positioning
  • Spending 15 to 30 minutes for the session, including the time it takes to put you in position and set up the equipment  

Multi-detector CT scanner

This scanner provides multiple image slices in one rotation, allowing radiologists to view high-quality images in micro-level detail and faster. The CT machine's wide scanning space makes it a more comfortable experience for the patient. Depending on the body part, this scan can be done in 10 to 30 minutes. If you need to be given contrast before your scan, it may take longer to set up.

PET/CT scan

This nuclear imaging technique uses both positron emission tomography (PET) and computerized tomography (CT), providing detailed information about the structure and function of cells and tissues in the body. The patient receives a glucose injection that has a small amount of radiation as a contrast agent. The radiation dose is absorbed by the organs or tissues being examined, making them visible in medical imaging. The radiation exposure from this procedure is minor.

Advantages may include:

  • Locating damaged or cancerous cells where the glucose is being abnormally absorbed (cancer cells often use more glucose than normal cells)
  • Determining the response to therapy by measuring the rate at which the tumor is using the glucose
  • Providing a more detailed image of cancerous tissues than either test is able to do on its own
  • Capturing both forms of diagnostic imaging in one scan
  • Spending about 20 minutes for the PET/CT scan (still, plan on extra time for prep and discharge) 

How is a CT scan performed?

A patient lies on a table that slides into the CT scanner, where an X-ray tube rotates around the patient. A technologist may inject a contrast material, usually an iodine contrast dye, into a vein in the arm for even more detailed CT images. Many abdominal and pelvic exams also require drinking a liquid to opacify the GI tract. This painless outpatient procedure typically takes about 10 minutes.

What to expect during a CT scan

Before the scan

  • Wear comfortable clothing. The technologist may ask you to change into a medical gown.
  • Remove metal objects such as jewelry, eyeglasses, dentures and hairpins. If you have piercings, remove them as well.
  • If you’re getting contrast, you may be asked to not eat or drink a few hours before the scheduled exam.
  • If you’re on medications, ask your doctor whether to take them before the exam.
  • If you have allergies, ask if there are medications to reduce the risk of a reaction to the contrast.
  • If you have other medical conditions, such as diabetes, heart disease, asthma, kidney disease, thyroid problems, or may be pregnant, tell your care team.

During the scan

  • A noninvasive procedure, a CT scan requires nothing to break the skin or enter the body. However, sometimes your doctor orders a CT with contrast dye to highlight specific areas of the body. In that case, the contrast agent may be injected into a vein or given orally or by enema.
  • You may be asked to hold your breath, so you don’t move and blur the images. You’re not enclosed like you are in an MRI machine, so it’s unlikely you’ll feel claustrophobic.
  • Most CT scans take anywhere from a few minutes to a half-hour and are available in different settings—whether that’s in the radiology department of a hospital or at an outpatient imaging facility.
  • CT scans are typically painless and easy, but if you’re given contrast through an intravenous (IV) line, you may experience slight discomfort at the injection site.
  • You may be asked to wait a few minutes for the technologist to check that the images are of the quality needed for an accurate interpretation.

Why is a CT scan used for cancer?

CT scans play many different roles in the diagnosis and treatment of cancer.

  • Screening: CT is sometimes used to screen for different types of cancer, such as lung and colorectal cancer.
  • Diagnosis: Your doctor may order a CT scan to locate and size suspected tumors. It also may help determine whether a tumor has recurred.
  • Planning and treatment guide: A CT scan may help your doctor identify and locate the tissue that needs to be biopsied. It may also be used to guide treatments such as cryotherapy, microwave ablation and the implantation of radioactive seeds, or to plan surgery or external-beam radiation.
  • Treatment response: Sometimes doctors order a scan to see whether a tumor is responding to treatment.
  • Tool for monitoring for other diseases: CT scans may be necessary to look for other conditions that may or may not be related to cancer, including:
    • Abnormal brain function
    • Coronary artery disease
    • Blood vessel aneurysms
    • Blood clots
    • Bone fractures
    • Emphysema or pneumonia
    • Kidney and bladder stones
    • Inflammatory diseases, such as ulcerative colitis and sinusitis
    • Injuries to your head or internal organs

Your treatment and type of cancer will determine how often you need to undergo follow-up with CT. For example, it’s recommended that patients treated with surgery for colorectal cancer have two CT scans within the first three years. If you’re between 55 and 74 years old and have a history of smoking an average of a pack a day for 30 years (even if you quit in the past 15 years), the American Cancer Society recommends you undergo a low-dose CT scan every year to monitor for lung cancer.

Side effects and risks with CT scan

As with any medical procedure, you and your doctor must weigh the risks and benefits.

The information your doctors may get from a CT scan may be critical to diagnosing and treating your cancer. Possible risks include:

Radiation. CT scans use low-level ionizing radiation. While it’s more radiation than emitted from an X-ray, it’s still low. Still, some concerns have been raised over whether even small doses of radiation from imaging may cause cancer. In most cases, the information obtained from the scan far outweighs the minimal risks of the radiation used. While the chance of CT scans causing cancer is small, several medical societies and government agencies have adopted guidelines and programs aimed at reducing this risk. 

Allergic reactions. Though it’s rare, sometimes people have allergic reactions to the contrast agents. You may experience itching or hives. If you have a serious allergic reaction, where you experience shortness of breath and your throat swells, alert the technologist immediately so that you may be treated promptly.

Kidney function. If your kidneys are impaired in any way, the contrast dye may make it worse. About 2 percent of patients who are given dyes develop contrast induced nephropathy (CIN), according to the National Kidney Foundation. CIN may cause fatigue, swelling of the feet and ankles, and dry, itchy skin. CIN also may lead to serious kidney and heart problems.


You may not be a candidate for a CT scan if you:

  • Are obese (usually more than 450 pounds)
  • Are pregnant 

Benefits of CT scans

  • The procedure is fast, simple, painless and accurate.
  • CT scans provide a wealth of information: Your doctors may see bone, soft tissue and blood vessels with one scan. They also get very detailed images of your tissues and organs.
  • Unlike MRI, CT scans may be performed on patients with implanted medical devices of any kind.
  • The information from a CT scan may eliminate the need for surgery or biopsy.
  • While it provides some radiation, no radiation remains in the body afterward.
  • A CT scan may be used to guide minimally invasive procedures such as needle biopsies and needle aspirations in areas of the body such as the lungs, abdomen and pelvis.

What do the results mean?

A radiologist analyzes your scans and sends a report to the physician who ordered the exam. Your doctor then discusses the results with you.

Depending on what’s found, the radiologist and your doctor may recommend additional or different imaging exams.

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