Can a CT Scan Detect Cancer? Accuracy, Types, and Limitations Explained

Introduction

When a doctor recommends a computed tomography (CT) scan, people often wonder, Can cancer be detected in a CT scan? A CT scan is commonly recommended when doctors need to get a detailed view inside the body to identify unusual symptoms or findings. It can also help identify abnormal growths¹. In this article, we will discuss CT scans, their uses, limitations, and more. 

What Is a CT Scan?

A CT scan is an imaging test that uses X-rays and computer technology to produce detailed, cross-sectional images of the inside of your body. Unlike a regular X-ray, which gives a single flat picture, a CT scan captures several images from different angles. A computer then combines these images to show detailed views of your organs, tissues, and other body structures. These images can be reconstructed to 3D images later, if needed. 

In some cases, doctors may use a contrast dye or material to make certain organs, blood vessels or unusual areas easier to see. Depending on the part of the body to be examined, the contrast may be given orally or through a vein. CT scans are commonly used to detect internal bleeding, infections, etc¹. 

Can a CT Scan Detect Cancer?

Yes, a CT scan can help doctors detect cancer by showing unusual changes inside the body. It can show abnormal growths, masses or tumours that may not be visible on a regular X-ray. CT images can also show the size, shape and location of a suspicious area, helping doctors understand where the abnormality is located². 

CT scans can examine different parts of the body, including the chest, abdomen, and pelvis. They may also help doctors check whether a tumour has affected nearby blood vessels or tissues, involved nearby lymph nodes, or metastasised (spread) to other areas or organs^2,3. However, getting an abnormal finding on a CT scan does not always mean it is cancer. Doctors may need other tests to determine the cause of the abnormality⁴. 

What Types of Cancer Can a CT Scan Detect?

A CT scan can help identify different types of cancer, such as: 

• Lung Cancer: Used to scan unusual spots, nodules or masses in the lungs and helps doctors assess their size and location. Low-dose CT is also used to screen for lung cancer in selected people at higher risk, such as those who meet specific age and smoking-history criteria^4,5. 
• Liver Cancer: Helps identify abnormal areas in the liver (using dye) and provides information about their size and position. Multiphase contrast CT, which involves taking images at different times after contrast dye is injected, may be used to better characterise liver lesions/abnormal areas in the liver⁶.
• Kidney Cancer: Can help reveal masses or other changes affecting the kidneys⁷.
• Pancreatic Cancer: Helps identify abnormalities in the pancreas and whether the cancer has spread to nearby tissues, lymph nodes or distant organs. Doctors usually consider a specialised type, pancreatic-protocol CT scan for suspected cases³.
• Colorectal Cancer: Gives detailed images of the colon and rectum (parts of the large intestine) and assesses the stage of known colorectal cancer by checking whether it has spread to nearby or distant areas (especially when combined with a positron emission tomography (PET) scan)⁴. 
• Stomach Cancer: Helps assess the extent of the stomach cancer after diagnosis by showing nearby lymph nodes and organs⁸.
• Ovarian Cancer: Helps identify large tumours in the ovaries and their spread to nearby organs, tissues, or lymph nodes. However, small tumours or peritoneal lesions may be missed⁹; ultrasound or magnetic resonance imaging  (MRI) may be needed to show these lesions more clearly in some cases. 

Note: A CT scan can help doctors spot areas that may need further checking, but it cannot always confirm whether a growth or abnormal area is cancer. Depending on the findings, doctors may recommend other tests, such as an MRI, a PET scan, blood tests, or a biopsy, to make an accurate diagnosis. 

Where Are CT Scans Less Effective?

Although CT scans are useful for detecting many cancers, they may not always provide clear results for every type of cancer. They can be less effective in certain situations, such as: 

• Very Small Tumours/Lesions: Tiny abnormalities, such as very small liver lesions, may not be clearly seen on the scan^9,10. 
• Soft Tissue Cancer: MRI can provide better detail for particular soft-tissue abnormalities, such as tumours affecting the connective tissue, muscles, and nerves, compared to a CT scan¹¹.
• Brain Cancer: CT scans have a limited role in detecting certain brain tumours and may not provide as much detail as an MRI, particularly for small abnormalities^12,13.
• Blood Cancers: CT scans may have limited ability to detect blood cancers such as leukaemia itself, as these do not form solid lesions and primarily affect the bone marrow. Leukaemia is mainly diagnosed by blood count examination, flow cytometry, and mainly by bone marrow examination. In other cases, such as lymphoma, PET-CTs are preferred¹⁴.
• Changes that Resemble Benign Conditions: Some cancerous and non-cancerous abnormalities can look alike on imaging¹⁵. For example, primary lung cancer can sometimes resemble a benign nodule or infection in a CT scan¹⁶. 

How Accurate Is a CT Scan for Cancer?

There is no single accuracy percentage for every CT scan. Its ability to detect cancer depends on several factors, including the type and stage of cancer, tumour size, location, scan quality and whether contrast dye is used^1,10,17. Very small cancers may be difficult to identify, while larger or more developed tumours may be easier to detect¹⁰. The use of contrast material can also improve the visibility of abnormalities in certain organs¹. 

A CT scan alone usually cannot confirm that an abnormal area is cancerous. Depending on the findings, a doctor may recommend additional tests such as an MRI, PET scan, blood tests, or a biopsy. These added tests can provide more information and, in the case of a biopsy, help confirm the diagnosis by examining cells or tissue under a microscope¹⁸.

Limitations and Considerations

CT scans offer valuable information, but they also have some limitations which you should know about: 

• False-Positive Results: A scan may show an abnormality that appears suspicious but later turns out to be harmless⁴. 
• False-Negative Results: A CT scan may miss some cancer-related changes due to factors such as small lesions/tumours, less soft tissue contrast (especially when compared to MRI), infiltrative disease, mucosal lesions or unclear lesions (such as with bone marrow), or use of an unsuitable CT scanning protocol^9,11,19.
• Limited Ability to Confirm Cancer: Imaging can identify suspicious changes, but they may not always confirm whether a mass is cancerous¹⁵. 
• Reduced Image Quality in Some Cases: Movement or certain technical factors can affect how clearly an abnormality appears²⁰.

Preparing for a CT Scan

Getting ready for a CT scan depends on the area being examined and whether contrast material will be used. Your doctor or scanning centre may give you instructions based on the type of scan you are having. 

• Follow Fasting Instructions: You may be asked to stop eating or drinking for a few hours before the scan, if needed²¹.
• Tell Your Doctor if You Are Pregnant: Let your doctor know if you are pregnant or think you could be pregnant. CT scans use X-rays, which expose the body to a small amount of radiation. 
• Report Allergies: Tell your doctor if you have had any allergy/reaction to CT contrast (iodinated) dyes. 
• Remove Metallic Objects: You may be asked to remove jewellery, glasses or other metal items, as they can interfere with imaging¹. 

Note: You might hear certain sounds during the scan, which is totally normal and shouldn’t be worried about. The procedure is usually painless. You can drink more fluid in case you were subjected to any dyes during the procedure, to flush them out. 

Common CT Scan Side Effects and Risks

Most people tolerate CT scans well. However, some possible risks or side effects that should be considered include the following: 

• CT scans use X-rays, which expose the body to high-energy radiation. However, doctors consider the possible benefits and risks before recommending a CT scan¹.
• As CT scans use radiation, there can be some risk to the developing foetus if done for pregnant women, depending on the part of the body being scanned. Head or chest CT scans generally result in very little foetal exposure, while scans of the abdomen or pelvis may expose the foetus to more radiation. However, a clinically necessary CT scan should not automatically be avoided during pregnancy; doctors weigh the benefits and risks before recommending a CT scan and may choose ultrasound or MRI if needed²². 
• Some people may experience temporary nausea after receiving contrast. 
• Contrast dye can also cause allergic reactions, such as a rash, an itchy or scratchy throat, etc. Severe reactions like wheezing or shortness of breath should be addressed right away. 
• Some people might experience temporary taste changes after a CT scan due to contrast dye. 
• In some people, especially those with significant kidney problems, certain contrast agents may create additional kidney problems².

The Importance of CT Scans in Cancer Detection

A CT scan can be an important tool for cancer detection. It offers the following advantages: 

• Helping with Early Detection: CT scans can identify unusual areas or growths in the body at an early stage in specific situations. For instance, people who have a higher risk of lung cancer may be given a low-dose CT scan to look for early signs of the disease. 
• Understanding the Cancer Stage: If cancer is diagnosed, a CT scan, along with a PET scan, can show the tumour’s size and help doctors check whether it has reached nearby tissues or other parts of the body. These findings can help doctors determine the cancer’s stage⁸.
• Planning Treatment: Knowing the location of the cancer and whether it has spread can help doctors decide on the right treatment approach. CT images may also be useful when planning procedures such as surgery and radiation therapy, as well as other cancer treatments³.
• Checking and Monitoring the Treatment Progress: Doctors may perform follow-up CT scans during or after treatment to check how the tumour is responding and whether it has become smaller or changed over time². 
• Rapid and Non-Invasive Technique: CT scans can produce detailed images of the body relatively quickly and being an imaging method, do not involve any surgical intervention. This can help doctors evaluate suspected abnormalities promptly and make timely decisions about further testing or treatment. 

Also Read: Can Vitamin B12 Deficiency Be a Sign of Cancer?

Conclusion

A CT scan can show abnormal masses or growths and help assess their size and location. It can also help doctors check for lymph node involvement and determine whether cancer has metastasised to other organs. However, it cannot always confirm whether a growth is cancerous. Its ability to detect cancer depends on factors such as the cancer type, tumour size, location, and the person’s individual situation. Although CT scans provide detailed images, they have limitations, including radiation exposure and the possibility of inaccurate results. Your doctor may recommend additional tests and combine their results with the CT scan to confirm the diagnosis. 

Frequently Asked Questions (FAQs)

References

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