Sidedness: right-side vs. left-side colorectal cancer

• The first section is the ascending colon. It extends from the small intestine to a pouch called the cecum. The ascending colon travels up the right side of the abdomen.
• The transverse colon is the second section, and it continues across the abdomen from the right side to the left side.
• The third section is the descending colon, which extends down the left side of the abdomen.
• The fourth section is the sigmoid colon. It is the last portion of the colon, and it connects to the rectum* and anus.

*Note: Some discussions of tumor sidedness include the rectum as part of the left side. However, rectal cancer is often treated differently from colon cancer, so your care team may consider it separately when planning treatment.

Research suggests that there are biological differences between left-sided and right-sided colon cancers. These differences are related to tumor biology, including which biomarkers are present, how the cancer grows, and how it responds to treatment.

In general, left-sided colorectal cancers tend to have a better overall prognosis than right-sided cancers, particularly in metastatic disease. However, tumor location is only one piece of the picture. Today, treatment decisions are driven primarily by biomarker testing (such as KRAS, BRAF, MSI/MMR, HER2, and others), with tumor location helping guide certain therapy choices.

Knowing both your tumor location and your biomarkers helps you and your care team choose the most effective, personalized treatment plan.

Colorectal cancer that starts on the right side (Cecum, Ascending Colon, Right Half of Transverse Colon, Hepatic Flexure)

• Are more commonly diagnosed in women, older adults, and African American patients, but these are broad population trends — not individual rules — and anyone can develop either type
• Often develop from flat (sessile) polyps, which can be harder to detect early
• Are more commonly diagnosed at a later stage
• May cause anemia, fatigue, or hidden bleeding rather than bowel changes
• Are more commonly associated with Lynch syndrome
• Have higher rates of:
  • MSI-H/dMMR
  • BRAF V600E mutations
  • KRAS mutations
  • Mucinous or signet-ring features
• Respond particularly well to immunotherapy when MSI-H/dMMR is present
• Are less likely to benefit from anti-EGFR targeted therapies in the first-line metastatic setting, even when RAS wild-type

Colorectal cancer that begins on the left side (Splenic Flexure, Descending and Sigmoid Colon, Left Half of Transverse Colon, Rectosigmoid, Rectum)

• Occur more often overall than right-sided cancers
• Are seen more often in men and in people under age 65
• Tend to form ring-like tumors that grow along the colon wall
• More commonly cause bowel habit changes, such as constipation, narrow stools, or obstruction
• Are more likely to be diagnosed at an earlier stage
• Are associated with familial adenomatous polyposis (FAP)
• Have higher rates of:
  • HER2 amplification (sometimes)
  • EGFR-pathway sensitivity when RAS/BRAF wild-type
• Respond better to standard chemotherapy
• In metastatic disease, patients respond better to anti-EGFR targeted therapies (such as cetuximab or panitumumab) when biomarkers are appropriate
• Have an overall better prognosis compared with right-sided tumors