Can CT Scans Detect Colon Cancer?

Colon cancer is currently likely to affect 1 in 24 patients assigned male at birth (AMAB) and 1 in 26 patients assigned female at birth (AFAB). While overall colon cancer rates are declining, cases among younger patients are rising, making early diagnosis and awareness all the more important.

Fortunately, early diagnosis has a high cure rate, and the medical community is increasingly recognizing the importance of preventative and diagnostic screenings such as colonoscopies and computed tomography (CT) scans. This article will focus on the use of CT scans in detecting colon cancer, including their strengths and limitations.

What is colorectal cancer?

Colorectal cancer (sometimes called bowel cancer) is a term that refers to either colon or rectal cancer and is often used interchangeably. The difference lies in location—colon cancer develops in the main portion of the large intestine, while rectal cancer is confined to the last 12 centimeters, known as the rectum.

These cancers occur when cells in the large intestine divide and spread uncontrollably, often forming growths called polyps. If left untreated, colorectal cancer can disrupt digestive function and organ health, and in advanced cases, spread to other parts of the body. Screening for polyps is one of the main reasons colonoscopies are currently recommended for patients aged 45 to 75. However, it’s not uncommon for benign polyps to be found during these screenings—and only 5 to 10 percent eventually become cancerous.

Colon cancer diagnosis

There are several methods that help medical providers confirm or rule out a colon cancer diagnosis. Many of these are used together, depending on the patient’s symptoms and health history:

Blood tests: A range of blood tests may suggest the presence of colorectal cancer. These can include liver enzyme tests—since cancer may spread to the liver—tumor marker tests that detect substances produced by cancer cells and tests for anemia, which can result from bleeding polyps.
Stool tests: Because polyps can bleed, the presence of blood in the stool may be a sign of colorectal cancer. When the blood isn’t visible to the naked eye, it’s referred to as “occult blood.” Stool samples can be tested for occult blood and other markers that may indicate colon cancer.
Colonoscopy: Considered the gold standard for colorectal cancer screening and diagnosis, a colonoscopy uses a thin, lighted tube with a camera and small tools (called an endoscope), which is inserted through the anus and into the colon. The physician can examine the colon, remove polyps (if they aren’t too large) and take biopsies of any suspicious areas for further testing.
MRI: Magnetic resonance imaging uses strong magnets and radio waves to produce highly detailed images of the body. It can help detect tumors and growths, and determine whether cancer has spread.
PET scan: Positron emission tomography is an imaging technique that uses a low-dose radioactive sugar, which is absorbed by tissues in the body. A special camera tracks this absorption, since cancer cells absorb more of the sugar. The scan can highlight affected areas and show whether the cancer has spread to other regions.
Other imaging tests: Additional imaging methods can be helpful for diagnosis. Ultrasounds, for instance, use sound wave echoes to create images of internal organs and structures for analysis. Similarly, CT scans use X-ray technology for this purpose. They can both assess whether colon cancer has spread to other organs or lymph nodes.
Biopsy: This involves removing tissue samples from a polyp or a suspicious area of the colon and sending them to a laboratory for analysis. A biopsy can confirm the presence of cancer cells and provide important details about the cancer itself.

What is a CT scan?

CT scans are a medical imaging technique that uses low-dose X-rays to create detailed images of the inside of the body. Traditional X-rays produce only a few images—usually 2 to 4—while CT scans take numerous images, called slices, and compile them into a highly detailed, cross-sectional view with much greater clarity.

This imaging technique has a wide range of uses beyond detecting colorectal cancer, including assessing injuries, identifying kidney stones, detecting signs of disease or abnormalities and even guiding surgical or medical procedures in real time.

CT scans and virtual colonoscopies

A CT colonography scan

In the past, medical imaging was typically not used for initial detection of colon or rectal cancer, but to determine whether the disease had spread to other parts of the body. Traditional colonoscopies were, and still are, the most common method for diagnosing colorectal cancer. However, computed tomography is now being used more frequently as a diagnostic tool, thanks to its ability to perform virtual colonoscopies, also called a CT colonography.

Because CT scans can view the body in multiple planes, they can be used to create 3D images, including of the colon and large intestine. A regular colonoscopy is an invasive procedure that involves inserting an endoscope through the anus and into the colon. With a virtual colonoscopy, this step isn’t necessary. Both types of colonoscopy require bowel preparation to ensure the colon is empty for the exam, but the virtual version does not use an endoscope, making it easier to perform and less invasive than the traditional method. If there are any findings during the scan, such as a polyp, additional testing will be performed.

Strengths of virtual colonoscopies

Screening for polyps in early stages is an important component of preventing or treating colorectal cancer. If they are found in these early stages, they can be surgically removed before the cancer develops. Here are some strengths of using a CT scan to look for polyps and other signs of colon cancer:

Doesn’t require sedation: Because it is non-invasive, this appointment doesn’t require sedation, meaning the patient can drive to and from the appointment themselves.
Nearly as detailed as traditional colonoscopies: Polyps, lesions and other abnormalities can be seen in nearly as much detail as with a regular colonoscopy.
Less risky for certain patients: For patients with underlying conditions that make sedation or endoscope insertion riskier, a CT scan is a safer alternative.
Lower risk of perforation: Traditional colonoscopies carry a small risk of colon perforation, which is reduced with the CT scan version.
Can detect anomalies outside of the colon: Areas outside the colon can also be seen, allowing for potential detection of other conditions and diseases.
Faster recovery times: Because sedation or pain medications aren’t needed, most patients can resume normal activities after the appointment.
Can be used with bowel obstructions: When patients have obstructions, such as tumors or intestinal narrowing, the scan can still be performed—unlike a traditional colonoscopy.
Less expensive: For most patients, the CT scan is less expensive and easier to schedule than a traditional colonoscopy.

Limitations of virtual colonoscopies

Despite the effectiveness of CT colonoscopies, they do have some limitations, including:

Diagnostic-only procedure: CT scans can diagnose the presence of polyps or abnormalities but cannot remove them or take tissue samples for analysis. If a polyp is detected, the patient may need a traditional colonoscopy at a later date to remove it.
Smaller issues may not be detected: Very small polyps or abnormalities may not be visible on a CT scan.
Size constraints: Many CT scanning machines have weight limits that may not accommodate all patients.
Not appropriate for some underlying conditions: Patients with Crohn’s disease, inflammatory bowel disease, similar digestive conditions or high risks for colon cancer are often not recommended for a CT colonography.
Not always covered by insurance: Some insurance plans do cover this exam, but coverage depends on the provider and the medical reason for the procedure.
More frequent screenings: Because CT scans may miss small polyps, they are recommended more often than traditional colonoscopies—every 5 years for CT scans compared to every 10 years for a full colonoscopy.

CT scans and colon cancer staging

Besides serving as an alternative to a traditional colonoscopy, CT scans canare also used to detect colorectal cancer in other areas of the body. This helps determine staging, which describes how much the cancer has grown and spread. Knowing the stage of the cancer can guide treatment decisions.

Colorectal cancer staging can be highly detailed and complex, but the basic stages consist of:

Stage 1 and 2: Cancer cells have not spread beyond the colon wall.
Stage 3: Cancer cells have spread to nearby lymph nodes.
Stage 4: Also called metastatic cancer, the disease has spread to distant regions of the body.

CT scans can also be used to help guide biopsies of organs such as the liver and lungs to determine if colon cancer has spread to these areas. Real-time imaging during the scan allows the doctor to precisely locate the area being biopsied, helping ensure accuracy during the procedure.

How is a CT scan for detecting colon cancer performed?

Preparation for a CT scan depends on whether it is being used for a CT colonography or to help stage possible cancer or assess organ health. For a colonography, the patient will need to complete bowel preparation beforehand to ensure the intestines are empty and do not obstruct the view. A small tube may also be inserted into the rectum during the exam to inflate the colon for better visibility. Aside from these steps, most CT scans—regardless of their purpose—are performed in the following way:

The CT scan machine is shaped like a large donut, with an exam table that moves in and out of the hollow, tube-like section of the equipment. The patient will lie on the table as it is positioned for the scan. Often, the patient will wear a hospital gown.
During the scan, the imaging technician operates the machine remotely from a separate area, though they can usually communicate with the patient via speaker. The rotating X-ray tube moves around the table to capture images.
Some CT scans require a contrast dye, which is injected into the patient’s arm. This dye helps differentiate organs, tissues and other areas in the scan, highlighting details more clearly. In some cases, the dye is administered halfway through the exam to capture images with and without it, while in other cases, it is given at the beginning.
During the scan, patients may be asked to shift positions or hold their breath for short periods to obtain specific views of internal structures. Most of the scan will require them to stay still.
Once the scan is finished, the rectal tube (if applicable) will be removed, and patients can resume their day. A colonography typically takes about 15 minutes, but if the CT is being used for detecting and staging colon cancer, the appointment may take longer.

How to prepare for a CT scan

Here are some tips to keep in mind for CT scan preparation:

Ask about bowel preparation (if applicable): If you’re having a colonography, bowel preparation is required before the appointment. This usually involves following a special diet for a certain period and having only liquids the day before or the day of the appointment. Instructions may vary by facility, so ask for specific recommendations.
Inform your doctor if you’re pregnant: If you think you may be pregnant, be sure to inform your doctor in advance. Many physicians recommend avoiding CT scans during pregnancy unless there is a medically necessary reason.
Inform your doctor of medication allergies: If you have any known medication allergies, especially to contrast dye, let your doctor know ahead of time.
Bring your requisition form: It’s helpful to bring your requisition or CT scan form, in case the facility doesn’t have it on hand. Be sure to also bring your health or insurance card and a valid ID.
Wear clothing that’s easy to change out of: Most facilities will require you to wear a hospital gown, but depending on the type of CT scan, you may not need one. Regardless, wear a two-piece outfit that is easy to remove. Also, try to avoid wearing metal if possible.

Getting your results

Turnaround times for imaging results can vary widely depending on the facility and your doctor’s availability. Often, patients wait a week or more and receive their results during a follow-up appointment. With PocketHealth, you don’t have to wait as long—your results are securely accessible as soon as the report is available, allowing you to review them often before your follow-up visit.

Your imaging results are interpreted by a radiologist—an expert in medical imaging—who carefully reviews your scans and provides a detailed report of any findings. This report is then sent to your referring doctor, who, with a deeper understanding of your medical history, can offer further insights and recommendations.

To better understand your imaging reports, Report Reader provides clear, straightforward definitions for medical terms. Simply tap or click on any underlined words to reveal their meaning. This is paired with illustrations and highlights of anatomy in your imaging to help you better understand your results and prepare for follow-up appointments.

Frequently asked questions

Here are some frequently asked questions regarding CT scans and colon cancer.

What are the risks of CT scans?

In a virtual colonoscopy, there is a very small risk of bowel perforation when the colon is inflated, though this occurs in fewer than 1 in 10,000 cases. As for the CT scan itself, it involves low-dose radiation, which has been linked to cancer with excessive exposure. Because of this, patients with a history of frequent radiation exposure may not be ideal candidates unless their doctor considers the benefits to outweigh the risks. For most patients, however, occasional exposure is considered to be within safe limits.

How accurate are CT scans?

CT scans are highly accurate and detailed, making them well-suited for assessing internal health and evaluating disease progression, injuries and other medical conditions.

What are the early signs of colon cancer?

Possible symptoms of colorectal cancer may include:

Unusual bowel changes: Examples include more constipation or diarrhea than is typical for the patient.
Fatigue: Feeling tired or weak without explanation.
Unintended weight loss: Unintentionally losing weight.
Abdominal discomfort: Pain, cramps and other symptoms.
Bloody stool: Blood found in the feces or rectal bleeding. Dark colored feces may also be a sign of blood.

It’s important to remember that many benign conditions can cause symptoms similar to those of colon cancer. Irritable bowel syndrome, lifestyle factors and other health issues often share these signs. If you have concerns, consider discussing them with your doctor to determine the appropriate next steps and receive personalized guidance.

What are the guidelines for colon cancer screening?

Current guidelines from the US Preventive Services Task Force recommend that patients aged 45 to 75 undergo regular colorectal cancer screenings, such as a colonoscopy. After age 75, screening decisions should be made based on a patient’s individual health and their doctor’s advice. These screenings are typically done every ten years, as polyps usually take at least that long to become cancerous.

For patients with pre-existing conditions such as Crohn’s disease, ulcerative colitis, or inflammatory bowel disease, their screening test may begin before age 45 or occur more frequently than every ten years. Other risk factors, like a family history of colon cancer, can also influence when and how often screenings are needed.

Getting support

Receiving a colon cancer diagnosis can be an overwhelming experience. Many patients find it helpful to connect with support groups or access cancer-related resources during treatment and recovery. Your doctor can likely provide local options, but online resources can also offer valuable support, including:

Take control of your health journey

PocketHealth makes it simple to keep track of your colorectal imaging. All of your vital imaging is in one secure location and can be accessed online anytime. Reports can also be easily shared with other physicians in your care team if needed. Additionally, it makes it easy to track health changes over time, such as with repeat imaging. When used in conjunction with your medical provider’s professional advice, it is a powerful tool for organizing and understanding your imaging results and your health.

PocketHealth MyCare Navigator gives personalized insights into your health and identifies any recommended follow-up steps. This feature can also generate individualized questions to ask your doctor based on the findings in your report, ensuring you make the most of your consultation. A colon cancer diagnosis may feel overwhelming, but working closely with your providers can give you the best chance of managing your condition and protecting your health.