Understanding Colon Cancer

Colon cancer is classified by the type of cells where the cancer begins. The most common type by far is adenocarcinoma, which accounts for 90 to 95 percent of all colon cancer cases. Adenocarcinoma starts in the glandular cells that line the inside of the colon and produce mucus. Less common types include:

• Carcinoid tumors, which form in hormone-producing cells of the intestine
• Gastrointestinal stromal tumors (GISTs), which start in the muscle layers of the colon wall and can be benign or malignant
• Lymphomas, which are cancers of the immune system that can begin in the colon
• Sarcomas, which are rare and start in the connective tissues of the colon wall

Some colon cancers are linked to inherited genetic conditions:

• Familial adenomatous polyposis (FAP) causes hundreds of polyps to develop in the colon and rectum, often turning cancerous by age 40 if not treated
• Lynch syndrome, also called hereditary nonpolyposis colorectal cancer (HNPCC), significantly increases the risk of colon cancer and other cancers

Colon cancer is staged from 0 to IV based on how far it has spread. Stage 0 is confined to the inner lining, while stage IV means the cancer has spread to distant organs such as the liver or lungs.

Colon cancer is often called a silent disease because it frequently produces no symptoms in its early stages. This is why regular screening is so important — it can detect cancer long before you feel anything is wrong. When symptoms do appear, they can vary depending on the size and location of the tumor. Common symptoms include:

• A change in bowel habits that lasts for more than a few weeks, such as diarrhea, constipation, or a change in stool consistency
• Blood in the stool, which may appear bright red or dark and tarry
• Rectal bleeding
• Stool that is narrower or thinner than usual
• A feeling that your bowel does not empty completely after a bowel movement
• Persistent abdominal discomfort, such as cramps, gas, or pain
• Unexplained weight loss
• Fatigue or weakness
• A lump or mass in the abdomen

Many of these symptoms can also be caused by less serious conditions such as hemorrhoids, irritable bowel syndrome, or infections. However, if you experience any of these symptoms for three weeks or longer, it is important to see a doctor for evaluation. Because colon cancer can grow slowly over many years without causing noticeable symptoms, routine screening is the most effective way to detect it at an early, highly treatable stage.

Colon cancer begins when healthy cells in the lining of the colon develop changes in their DNA. These genetic mutations cause cells to grow and divide uncontrollably, eventually forming a tumor. The process from a normal cell to a cancerous one typically takes many years, which creates a window of opportunity for prevention through screening. The vast majority of colon cancers start as polyps — small, benign growths on the inner surface of the colon. The most common type of precancerous polyp is an adenoma (adenomatous polyp). Over time, some adenomas can become cancerous. This process typically takes years, which is why regular screening and polyp removal are so effective at preventing colon cancer. Several factors can increase your risk of developing colon cancer:

• Age: The risk increases significantly after age 50, though cases in people under 50 are rising
• Family history: Having a close relative with colon cancer or polyps increases your risk
• Personal history: A previous colon cancer or polyps raises the likelihood of developing new ones
• Inflammatory bowel disease: Conditions such as Crohn’s disease and ulcerative colitis increase risk
• Inherited genetic syndromes: FAP and Lynch syndrome are the most common
• Obesity: Excess body weight is linked to higher risk
• Physical inactivity: A sedentary lifestyle is associated with increased risk
• Smoking: Tobacco use increases the risk of many cancers, including colon cancer
• Heavy alcohol use: Regular, excessive alcohol consumption raises risk
• Diet: A diet high in red and processed meats and low in fiber is associated with higher risk
• Type 2 diabetes: People with diabetes have an increased risk
• Race: Colon cancer rates are highest among American Indian, Alaska Native, and Black populations in the United States

Having one or more of these risk factors does not mean you will develop colon cancer, but it does mean that regular screening is especially important for you.

Colon cancer is most often detected through routine screening, which is recommended starting at age 45 for people at average risk. Screening is crucial because colon cancer often has no symptoms in its early stages. Several screening tests are available:

• Colonoscopy is the gold standard. A thin, flexible tube with a camera is used to examine the entire colon. Polyps can be removed during the procedure. It is recommended every 10 years.
• Fecal immunochemical test (FIT) checks for hidden blood in the stool and is done yearly at home.
• Stool DNA test (such as Cologuard) detects blood and genetic markers from cancer cells in the stool, done every 3 years.
• Flexible sigmoidoscopy examines the lower part of the colon and is recommended every 5 years.
• CT colonography (virtual colonoscopy) uses X-rays to create images of the colon and is done every 5 years. If polyps are found, a standard colonoscopy is still needed.
• The Shield Blood Test was approved by the FDA in July 2024 as a blood-based screening option.

If screening finds polyps or suspicious tissue, a biopsy is taken during colonoscopy and examined under a microscope. If cancer is confirmed, additional tests help determine the stage:

• CT scans of the chest, abdomen, and pelvis check for spread to other organs
• MRI may provide more detailed images
• A blood test for carcinoembryonic antigen (CEA) can help monitor the disease

The stage of the cancer — from 0 to IV — determines the treatment approach and helps predict the outlook.

Colon cancer is one of the most preventable types of cancer, primarily because it develops slowly from precancerous polyps that can be found and removed during screening. The single most important preventive step is to get regular screening starting at age 45. Colonoscopy can detect and remove polyps before they ever have a chance to become cancerous. Beyond screening, several lifestyle measures can lower your risk:

• Eat a high-fiber diet rich in fruits, vegetables, whole grains, and legumes. Fiber helps move waste through the colon more quickly, reducing the time that potential carcinogens are in contact with the intestinal lining.
• Limit red and processed meats, such as beef, pork, bacon, sausage, and deli meats. These have been consistently linked to a higher risk of colon cancer.
• Reduce dietary fat, especially saturated fats from animal sources.
• Maintain a healthy body weight. Obesity is a well-established risk factor.
• Exercise regularly. Aim for at least 150 minutes of moderate physical activity per week.
• Do not smoke. Smoking increases the risk of many cancers, including colon cancer.
• Limit alcohol. If you drink, do so in moderation — no more than one drink per day for women and two for men.
• Some studies suggest that calcium and vitamin D may have a protective effect, and aspirin may reduce risk in certain people. Discuss these options with your doctor before starting any supplement or medication.

For people with a strong family history of colon cancer or known genetic syndromes, more intensive screening starting at a younger age may be recommended. Talk to your doctor about your personal risk profile.

The outlook for colon cancer depends heavily on the stage at which it is found. When detected early, colon cancer is highly treatable and the prognosis is excellent. Five-year relative survival rates for colon cancer:

• Stage 0 and stage I: 91 percent
• Stage II and stage III: 73 percent
• Stage IV (distant spread): 13 percent
• Overall (all stages combined): approximately 63 percent

These figures mean that people who are diagnosed at an early stage have a very high likelihood of surviving at least five years after diagnosis. Survival rates have been improving steadily over time due to advances in screening, surgical techniques, chemotherapy, and newer treatments such as immunotherapy and targeted therapy. One concerning trend is the rising incidence of colon cancer in younger people. Between 2012 and 2021, cases increased by about 2.4 percent per year among people under 50. This is why screening guidelines were recently lowered from age 50 to 45, and why anyone experiencing persistent digestive symptoms should see a doctor regardless of their age. After treatment, regular follow-up care is essential. This typically includes:

• Colonoscopy within 6 to 12 months after surgery
• CT scans of the chest, abdomen, and pelvis every 6 to 12 months for the first few years
• CEA blood tests every 3 to 6 months for up to 5 years
• Regular physical exams

With early detection, appropriate treatment, and consistent follow-up, the majority of people diagnosed with colon cancer can expect a favorable outcome.

Treatment for colon cancer depends on the stage of the disease, the location of the tumor, and the person’s overall health. A multidisciplinary team typically works together to develop a personalized treatment plan. Surgery is the most common treatment for colon cancer. For early-stage cancers confined to a polyp, removal during colonoscopy (polypectomy) may be sufficient. For larger tumors, surgery to remove the affected part of the colon (partial colectomy) along with nearby lymph nodes is usually needed. In most cases, the remaining sections of the colon can be reconnected, and a colostomy is rarely required for colon cancer. Chemotherapy uses drugs to kill cancer cells or stop them from growing. It may be given after surgery to destroy any remaining cancer cells (adjuvant chemotherapy), which is common for stage III colon cancer. It may also be given before surgery to shrink a tumor (neoadjuvant chemotherapy) or as the primary treatment for advanced disease that cannot be surgically removed. Radiation therapy uses high-energy beams to kill cancer cells. It is more commonly used for rectal cancer than colon cancer, but may be used in certain situations for colon cancer as well. Immunotherapy helps your immune system recognize and attack cancer cells. Checkpoint inhibitors such as pembrolizumab (Keytruda) and nivolumab (Opdivo) can be highly effective for colon cancers that have specific genetic markers, such as high microsatellite instability (MSI-H). Targeted therapy uses drugs that attack specific proteins or genes that help cancer cells grow. These are often used for advanced colon cancer and may be combined with chemotherapy. Examples include bevacizumab (Avastin), cetuximab (Erbitux), and panitumumab (Vectibix). Ablation and embolization are procedures used to treat colon cancer that has spread to the liver. They destroy tumors using heat, cold, alcohol, or by blocking the blood supply to the tumor. Clinical trials may offer access to new and emerging treatments for people with advanced colon cancer. After treatment, regular monitoring with colonoscopy, CT scans, and blood tests is essential to detect any recurrence as early as possible.

Diet plays a significant role in colon cancer prevention and can also support recovery during and after treatment. Foods that may help lower your risk of colon cancer:

• High-fiber foods such as fruits, vegetables, whole grains, beans, lentils, and nuts. Fiber helps move waste through the digestive system more quickly and may help remove cancer-causing substances from the colon.
• Cruciferous vegetables such as broccoli, cauliflower, cabbage, and Brussels sprouts contain compounds that may help protect against colon cancer.
• Calcium-rich foods such as low-fat dairy, fortified plant milks, and leafy greens may offer protective benefits.
• Whole grains such as oats, brown rice, quinoa, and whole wheat bread are excellent sources of fiber and nutrients.
• Fish, particularly fatty fish like salmon, mackerel, and sardines, provides omega-3 fatty acids with anti-inflammatory properties.

Foods to limit:

• Red meat (beef, pork, lamb) and especially processed meats (bacon, sausage, hot dogs, deli meats) have been consistently linked to an increased risk of colon cancer. The World Health Organization classifies processed meat as a carcinogen.
• Highly processed foods and foods high in saturated fats.
• Sugary drinks and refined carbohydrates.
• Excessive alcohol.

During treatment, nutritional needs may change. Some people experience nausea, appetite loss, or changes in taste from chemotherapy or radiation. Working with a registered dietitian can help you maintain adequate nutrition during this time. After colon surgery, smaller, more frequent meals may be easier to digest. Some people may need to avoid certain high-fiber foods temporarily and gradually reintroduce them. Staying well hydrated is especially important. Maintaining a healthy weight through diet is one of the most effective ways to reduce your risk of colon cancer and improve outcomes if you have been diagnosed.

Colon cancer begins when abnormal cells grow uncontrollably in the lining of the large intestine, often developing from precancerous polyps over many years. It is the third most common cancer in the United States, but it is also one of the most preventable when detected early. The majority of colon cancers are adenocarcinomas, and the disease is staged from 0 to IV based on how far it has spread. Many people have no symptoms in the early stages, which is why regular screening starting at age 45 is critical. Colonoscopy is the gold standard screening tool because it can both detect and remove polyps before they become cancerous. Treatment depends on the stage and may include surgery, chemotherapy, immunotherapy, targeted therapy, or a combination. The five-year survival rate for early-stage colon cancer is 91 percent, highlighting the importance of early detection. A diet rich in fiber, fruits, vegetables, and whole grains, along with regular exercise, maintaining a healthy weight, not smoking, and limiting alcohol and red meat can all help reduce the risk. With timely screening, appropriate treatment, and consistent follow-up care, most people diagnosed with colon cancer have an excellent prognosis.