The Accuracy Of CT Scans For Cancer Detection

How Accurate Is a CT Scan for Cancer Detection?

A CT scan - also known as a CAT scan or computed tomography scan - is often used to help diagnose cancer. But how accurate is a CT scan for cancer detection?

In general, CT scans are a highly accurate tool for detecting many different types of cancer, though not all.

This is because CT scans create highly detailed 3D images of the inside of the body, allowing them to detect the size, shape and precise location of tumours. However, a tissue sample (biopsy) will still need to be collected for an accurate diagnosis of cancer.

Here, we will explore how CT scans work and detect cancer, and how they compare to other medical imaging tools. We will also cover the types of cancers that CT scans are not as effective at detecting.

Understanding CT scan technology

CT scans use X ray technology and computer technology to create detailed images of the inside of your body.

A CT scanner has several main parts: a computer, a console, a sliding examination table and a ring-shaped gantry that contains an X-ray generator and X-ray detector.

To perform a CT scan, a patient lies down on the examination table, which is then passed through the gantry.

The X-ray generator produces a narrow beam of X-rays. This is different to a standard X-ray machine that produces a broad X-ray beam. The narrow beam of X-rays is directed to the specific part of the body that needs to be imaged.

As the gantry rotates around the patient, the narrow X-ray beam passes through the body at different angles. The X-ray detector is also located within the gantry and detects the X-rays that pass through the body without being absorbed. This information is converted into electrical signals and sent to a computer.

The computer digitally processes the information into a 2D cross-section of the body. Multiple 2D cross-sections are collected along the length of the parts of the body that need to be imaged. These detailed cross sectional images are then stacked together to form a 3D greyscale image.

Whiter parts of the image represent denser tissues, such as bone and blood clots. Other soft tissues that are less dense than bone appear different shades of grey, while air appears black.

How CT scans detect cancer

As CT scans can detect changes in a variety of soft tissues, they can detect many different types of cancer. This includes ovarian cancer in women, as well as:

bladder cancer
colorectal cancer and stomach cancer
liver cancers
lung cancer
kidney cancer
pancreatic cancer

The 3D nature of CT scans makes it easier to locate tumours and assess the extent of their growth from multiple angles.

CT scans can also detect the blood vessels supplying a tumour. This is helpful to determine how much the tumour has progressed and how aggressive it is.

Identifying tumour-supplying blood vessels is also useful for planning cancer treatment and checking if treatment is working.

Factors affecting CT scan accuracy

The clearer and more detailed a CT scan image is, the greater its accuracy in detecting cancer. Several factors affect the clarity of CT scan images, including:

Contrast

In some cases, your doctor may recommend having a CT scan with contrast (a type of dye). This means that immediately before your scan, you will be given a contrast agent. For CT scans, this is usually an iodine-based liquid dye.

The contrast agent is most commonly given as an injection into a vein or mixed with water to be given orally as a drink. However, in some cases, it may be given as an enema via your rectum. How the contrast material is administered depends on which part of your body needs to be scanned.

The contrast agent will enter your bloodstream and be taken up by your tissues. As it absorbs X-rays, areas containing higher amounts of contrast agent appear brighter on a CT scan. Tumours often take up more contrast agent and, consequently, can be easier to detect with a contrast-enhanced CT scan.

If you have previously had an allergic reaction to a contrast agent, you should tell your care team. They may suggest a different scan, such as a magnetic resonance imaging (MRI) scan.

It is important to note that allergic reactions to contrast agents are rare, and having an allergic reaction to one contrast agent does not mean you will have an allergic reaction to all contrast agents.

Movement

In order to create detailed 3D images, you need to remain very still during your scan. This is because any movement can cause blurry images.

In some cases, particularly when imaging the chest, you may be asked to temporarily hold your breath to help improve the quality of the scan images.

If you are feeling anxious about your CT scan, it can be challenging to remain still. It is helpful to let your care team know so that they can better support you.

Metal

When you have a CT scan, you will be asked to remove all metal items around the body part to be scanned. This is because metal reduces the quality of the images created by introducing streaks and other artefacts.

Metal implants that cannot be removed (eg dental implants and fillings, hip and knee replacements, stents, surgical staples, screws and plates) similarly reduce the clarity of CT scan images.

If you have any metal implants, you should inform your care team before your scan. Depending on the location of your implant and the area of your body that needs scanning, your CT scan may still be possible. In some cases, artefacts introduced by a metal implant can be cleared up using a single energy metal artefact reduction (SEMAR) algorithms.

Comparing CT Scans to Other Imaging Tests

CT scan vs X-ray

Compared to a standard X-ray, which produces 2D images, a CT scan produces much more detailed 3D images. This allows for the early detection of diseases.

However, a CT scan takes longer to complete, sometimes up to an hour, compared to an X-ray that takes a few minutes.

A CT scan is also more expensive and uses a higher dose of ionising radiation. For comparison, a chest X-ray uses 0.1 mSv of radiation while a chest CT scan uses 6.1 mSv.

However, the levels of radiation exposure for a CT scan are still considered to be low and consequently safe. For example, a brain CT scan uses 1.6 mSV of radiation, which is equivalent to 7 months of exposure to the radiation present naturally in the environment.

CT scan vs MRI scan

Although a CT scan produces highly detailed 3D images of the inside of the body, an MRI scan produces the highest resolution images of soft tissues, such as internal organs, muscles, ligaments and tendons.

While a CT scan is still often used to diagnose certain cancers, an MRI may subsequently be recommended after the initial diagnosis to check the extent of tumour growth at a higher resolution. MRI may also be used to assess the area after treatment to check for remaining tumour tissue.

Unlike a CT scan, an MRI scan does not use any ionising radiation. Instead, MRI uses radio waves and strong magnets. The use of strong magnets means MRI is not always suitable for individuals with certain metal implants.

An MRI scan takes longer to complete than a CT scan, sometimes taking up to 90 minutes. MRI can also be more challenging for those who experience claustrophobia as the scan involves being passed into an enclosed tube, rather than through a ring.

Limitations of CT scans in cancer detection

Despite the detailed images produced by CT scans, limitations in CT scan resolution mean they cannot detect very small tumours ie very early-stage cancers. This is particularly true for certain soft tissue cancers, including:

cervical cancer
ovarian cancer
prostate cancer

A CT scan may also be unable to provide the detail needed to fully assess the extent of tumour growth or whether any tumour remains after treatment - MRI scans are often better suited for these situations.

What’s more, although CT scans often play a key part in diagnosing cancer, a definitive diagnosis is usually only possible after a biopsy.

Your CT scan results

At Vista Health, we aim to provide you with a detailed report of your CT scan results within 3 working days. This report can also be sent to your NHS GP. In addition, you can book a consultation with one of our expert Vista Health GPs to discuss what your results mean and next steps.

Do you need a CT scan?

If you’re concerned about unexplained pain or discomfort, or are worried about your cancer risk due to your family medical history, speak to one of our experienced GPs.

They can determine whether a CT scan would help provide the insights you need. If appropriate, they can arrange for you to have a private CT scan at one of our conveniently located clinics nationwide.

Discover more about our private CT scan services today.