Understanding Your Diagnosis

What Is Colon Cancer?

Colon cancer starts in the cells that line the colon, also called the large intestine. The colon is the last part of the digestive system, responsible for absorbing water and forming waste before it passes to the rectum. Most colon cancers begin as small, benign growths called polyps that, over time, can develop into cancer.

The most common type is called adenocarcinoma, which grows from the gland cells lining the inside of the colon. There are other rarer types, but adenocarcinoma accounts for the vast majority of colon cancer diagnoses.

The location of the cancer within the colon matters. Right-sided colon cancers (ascending colon) and left-sided colon cancers (descending and sigmoid colon) can behave differently, have different molecular profiles, and may respond differently to treatment. This is worth asking your oncologist about specifically.

One trend worth naming: while colorectal cancer rates are falling in older adults, they continue to rise in younger adults (American Cancer Society, Colorectal Cancer Statistics 2026). If you are younger than the age most people associate with this diagnosis, you have every reason to advocate hard for thorough testing and a proactive plan. Do not wait.

Key Subtypes and Classifications

• Colon adenocarcinoma:The most common type. Starts in the gland cells lining the colon.
• Mucinous adenocarcinoma:Produces large amounts of mucus. Associated with MSI-H status. May behave differently than standard adenocarcinoma.
• Signet ring cell carcinoma:A rare, aggressive subtype. Important to know if this is the pathology.
• Stage I to II:Cancer is within the colon wall or just beyond it, but has not spread to lymph nodes.
• Stage III:Cancer has spread to nearby lymph nodes.
• Stage IV:Cancer has spread to distant organs. The liver and lungs are the most common sites.

 

The Most Important Data Point for Colon Cancer

MSI status and RAS/RAF mutations are the two most important molecular data points in colon cancer. They change treatment options, predict immunotherapy eligibility, and help your team understand the biology driving the cancer.

 

• MSI-H or dMMR:Microsatellite instability high or mismatch repair deficient. Opens the door to immunotherapy. Also associated with Lynch syndrome, a hereditary condition. If MSI-H is found, ask about genetic counseling.
• MSS or pMMR:Microsatellite stable. The most common molecular type in colon cancer. Standard chemotherapy is the foundation.
• KRAS and NRAS mutations:If mutated, EGFR inhibitors (cetuximab, panitumumab) will not work. Critical to know for metastatic disease.
• BRAF V600E mutation:Found in roughly 8 to 12 percent of colon cancers. Linked to more aggressive disease and right-sided tumors. When BRAF V600E is present in an MSI-H tumor, it usually means the cancer is sporadic rather than Lynch syndrome. For metastatic disease, adding encorafenib plus cetuximab to first-line chemotherapy (FOLFOX or FOLFIRI) is now a standard-of-care option that has roughly doubled median survival compared with chemotherapy alone (BREAKWATER trial, updated at ASCO 2026).
• HER2 amplification:An emerging target. Ask if HER2 was tested.
• NTRK fusion:Rare but highly actionable. Targeted therapy works very well if present.

 

A diagnosis is data. It tells you where you are starting. It does not tell you where you will finish. Data G’s use every piece of information to move forward with clarity.