How Colon Cancer Is Diagnosed

Getting an early diagnosis of colon cancer is of paramount importance to help improve outcomes - and getting that early diagnosis starts with following routine screening recommendations and knowing your family history.

Clinical guidelines recommend colon cancer screening begin at age 45 for people at average risk. Screening should begin sooner for anyone who has a family history or other risk factors. One method of screening, colonoscopy, can also serve as a diagnostic test if cancer is present.

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History and Physical Exam

Speaking with your doctor about your medical history is the first step in ensuring your risk for colon cancer is properly evaluated. Your doctor will talk through the risk factors that might be applicable to you, such as family history. The doctor will also talk to you about any symptoms you might be experiencing.

Your doctor will examine your abdominal area to check for masses or enlarged organs. You might have a digital rectal exam, during which your doctor inserts a lubricated, gloved finger into the rectum to check for abnormalities.

Colon Cancer Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

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Labs and Tests

After a medical history and physical examination, your doctor may order diagnostic tests, especially if your symptoms and/or exam results indicate that you could have a medical problem.

While labs cannot determine whether or not you have colon cancer, they can provide valuable information in your diagnosis.

Some blood tests your doctor may order include:

  • Complete blood count (CBC): This test can determine whether or not you have anemia (low red blood cell count), which is a potential complication of colon cancer due to bleeding from the tumor.
  • Liver function tests (LFTs): Since colon cancer may spread to the liver, your doctor may order these tests to assess how well your liver is functioning. 
  • Tumor markers: In some cases, colon cancer cells produce substances called "tumor markers" that can be detected through blood testing. This type of testing may be appropriate for monitoring cancer progression and treatment.

Diagnostic Colonoscopy

If your physical exam and/or blood tests indicate signs of possible colon cancer, your doctor will recommend more tests. A diagnostic colonoscopy is the most accurate test for diagnosing colon cancer. 

Colonoscopy Following Screening

If you had a stool-based test or CT scan that came back abnormal, your doctor would order a colonoscopy to confirm a diagnosis.

During a colonoscopy, a gastroenterologist—a doctor who specializes in treating diseases of the digestive tract—inserts a colonoscope (flexible tube) into your anus. You can watch on a video monitor as the camera is threaded through your rectum and colon. If you're thinking about how unpleasant that must be, take heart—you're sedated during the procedure. 

Biopsy

In addition, if a suspicious mass is seen in the colon, the doctor can take a biopsy (tissue sample). A pathologist can look at the sample with a microscope to see if cancer cells are present.

If cancer is found, more lab tests may be performed on the biopsied sample, like tests that look for gene changes in the cancer cells. Results of these tests can help oncologists (cancer doctors) determine what treatments may work best.

Imaging

Once the diagnosis of colon cancer is determined, the cancer stage, which is the extent of the disease spread, is determined with imaging tests. After the cancer has been staged, a treatment plan can then be devised.

Imaging tests that are often used include:

  • Chest X-ray
  • Computed tomography (CT) scan of the abdomen and pelvis (also known as a CT colonography or virtual colonoscopy)
  • Magnetic resonance imaging (MRI) of the liver
  • Positron emission tomography (PET) scan (not as commonly used) 

There are five stages of colon cancer (0–4) and, in general, the earlier the stage, the easier the cancer is to treat. 

Staging

To understand the basics of the stages, think of the colon as a hollow tube with five layers: the innermost layer (called the mucosa), the second layer (called the submucosa), a third muscular layer (called the muscularis propia), and the outermost layers (called the subserosa and serosa). 

colon cancer stage at diagnosis
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Stage 0

Stage 0 colon cancer is the earliest stage possible and is also called carcinoma in situ ("carcinoma" refers to cancer and "in situ" means original position or place). Stage 0 cancer has not grown beyond the mucosa. 

Stage 1

Stage 1 colon cancer means that the tumor has grown through the mucosa into the submucosa or the muscularis propia. 

Stage 2

Stage 2 colon cancer means one of the following scenarios:

  • Cancer has grown into the outermost layers of the colon, but not through them.
  • Cancer has grown through the outermost layer of the colon but has not grown into other nearby tissues or organs. 
  • Cancer has grown through the wall of the colon and attached to or grown into other nearby tissues or organs.
  • Cancer has grown through the mucosa into the submucosa and possibly the muscularis propia.

Stage 3

Stage 3 colon cancer means one of several things:

  • Cancer has grown into the submucosa layer and spread to four to six nearby lymph nodes. 
  • Cancer has grown into the outermost layers of the colon and spread to one to three nearby lymph nodes or into areas of fat near the lymph nodes.
  • Cancer has grown into the muscularis propia, or the outermost layer of the colon, and spread to four to six nearby lymph nodes.
  • Cancer has grown into the submucosa and possibly into the muscularis propia, and spread to seven or more nearby lymph nodes.
  • Cancer has grown through the wall of the colon and has spread to four to six nearby lymph nodes.
  • Cancer has grown into the outermost layers of the colon and spread to seven or more nearby lymph nodes.
  • Cancer has grown through the wall of the colon, is attached to or has grown into nearby tissues or organs, and has spread to at least one nearby lymph node or into areas of fat near the lymph node.

Stage 4

Like stage 2 and 3 colon cancer, there are a number of different scenarios that describe stage 4 cancer. Stage 4 colon cancer is synonymous with metastatic colon cancer, in which the tumor has spread to one or more distant organs (for example, the liver or lungs), to a distant set of lymph nodes, or to distant parts of the lining of the abdominal cavity (called the peritoneum).

Dealing with a diagnosis of stage 4 cancer can be a quite challenging process, both physically and mentally. For most people, stage 4 colon cancer is not curable, but there are usually treatment options available. 

Colorectal cancer

Survival Rate

Colon cancer survival rates hinge greatly on the stage of the disease. In essence, survival rates are used by doctors as a way to discuss a person's prognosis, which is the expected course of the disease. For example, when colon cancer is caught early before it has spread beyond the colon, the prognosis is very good, sometimes with a complete cure.

Five-Year Survival Rate

For early-stage cancers that are treated with the expectation of a complete cure, the five-year survival rate is sometimes considered the point at which a person is "out of the woods." After the five-year mark, it may become less likely for cancer to return. 

Some research suggests that people need to be followed closely for up to 10 years after diagnosis. This longer follow-up is to ensure that any cancer recurrences are caught early.

What Does "Five-Year Survival" Mean?

The five-year survival rate is the percentage of people diagnosed with cancer who are still alive at least five years after their initial diagnosis.

According to the National Cancer Institute and American Cancer Society, the five-year survival rates for people living with colon cancer (based on stage) are as follows:

  • Stage 1: 92%
  • Stage 2: 63 to 87%
  • Stage 3: 53 to 69%
  • Stage 4: 11%

Keep in Mind

It is important to remember that survival rates are estimates. A five-year survival rate can never predict what will happen in any individual case. Other things, such as how cancer responds to treatment and the genetics of the cancer cells, will affect the chances of survival.

In addition, to come up with five-year survival rates, health experts collect data about people who were treated for their cancer at least five years ago. Colon cancer treatments have changed and continue to change very quickly, which can improve outcomes.

Some treatments that are used now, such as targeted therapies, weren't available five years ago. Also remember that survival rates can include people who were diagnosed with colon cancer, but who later died of non-cancer-related causes.

This means that the five-year survival rates will likely look worse than what your five-year survival rate actually is. Be sure to discuss what you learn about five-year survival rates with your doctor. They can help you understand how this information might apply to your specific situation.

Differential Diagnoses

It's important to remember that one or more of the symptoms that may have brought you in to see your doctor (for example, rectal bleeding or abdominal pain) may be caused by another medical problem besides colon cancer.

That said, any new symptom should be evaluated, so a proper diagnosis and treatment plan can be initiated promptly.

Examples of other medical conditions that may mimic that of colon cancer include:

Hemorrhoids

Hemorrhoids are swollen veins in your anus or lower rectum that may cause painless bleeding during a bowel movement and/or discomfort in the anal area. 

Irritable Bowel Syndrome

Abdominal spasms and cramping are common in irritable bowel syndrome. 

Appendicitis

Appendicitis refers to inflammation of the appendix, which is a finger-like structure that sticks out from your colon. Appendicitis causes severe, often sudden pain around the umbilicus that moves toward the right lower side of the abdomen. Often, it causes nausea and/or vomiting and a loss of appetite. 

Diverticulitis 

Diverticulitis refers to inflammation of colon diverticulum (a pouch located in the colon wall). With diverticulitis, pain is often sudden, constant, and present in the left lower abdomen. Other associated symptoms include constipation, loss of appetite, nausea and/or vomiting. 

Infectious Colitis

Infectious colitis means that the colon is inflamed by an infection (for example, with the bacteria Clostridium difficile). This condition may cause diarrhea in addition to abdominal pain and a fever. 

Bladder Infection (Cystitis)

In addition to discomfort in the suprapubic region (the area located over your pubic bone), a person with cystitis may experience symptoms like increased frequency or hesitancy with urination or burning with urination.

Kidney Stone

A kidney stone often causes pain in the lower back that may radiate to the abdomen, in addition to blood in the urine.

Frequently Asked Questions

What tests can detect colon cancer?

Colonoscopy is the "gold standard" test, and flexible sigmoidoscopy (which only looks at part of the colon) is another option. A tissue biopsy can be obtained during one of these invasive diagnostic procedures. Imaging, such as CT colography, may detect growths. Lab tests can be used to detect molecular markers for cancer or analyze stool samples for blood.

Can a CT scan detect colon cancer?

Yes. Computed tomography (CT) imaging for the colon is called CT colography or, sometimes, a virtual colonoscopy. It may be used when someone can't tolerate a traditional colonoscopy or as part of the diagnostic process. Studies suggest CT colography isn't as effective as colonoscopy for detecting small polyps or very early stage colon cancer, although it may, in some cases, detect polyps situated behind folds of tissue. If a CT colography reveals the presence of polyps, they will need to be removed via a colonoscopy.

Can colon cancer show up on an ultrasound?

Ultrasound is mainly used to see if colon cancer has spread. Three types of ultrasound imaging may be used:

  • Abdominal ultrasound can detect the spread of cancer to the liver, gallbladder, or pancreas (but not the presence of tumors in the colon or rectum).
  • Endorectal ultrasound is performed by inserting the transducer (a thin instrument that picks up the sound waves) into the rectum to see if and how far cancer has infiltrated the walls of the rectum. It also can reveal if cancer has reached nearby organs or the lymph nodes.
  • Intraoperative ultrasound, which is done during surgery, can detect if colon cancer has spread to the liver.
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