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Side Matters: Left vs. Right

The colon is a 6-foot long, tube-like organ at the end of the digestive system that processes and prepares the removal of waste from the body. The colon is divided into the right side (which includes the cecum, ascending colon, and the hepatic flexure), the left side (which includes the splenic flexure, descending colon, sigmoid colon, rectosigmoid, and rectum) and the transverse colon. Additionally, the transverse colon is formed with the right colon, but it does not represent the characteristics of either the right or left and is not included in discussions of sidedness. Only 5% of colon cancers occur in the transverse colon.

Research suggests that there are biological differences between left-sided and right-sided colon cancers. This includes the biomarkers that are present on each side and the aggressiveness of the cancer. Patients with left-sided tumors typically have a better prognosis than those with right-sided tumors.

Make sure you know your biomarkers and where your tumor is located so that you and your doctor can choose the best treatment.

Cancer on the Right

More often diagnosed in women

Cecum, Ascending Colon, and Hepatic Flexure

  • Flat, sessile polyps go unnoticed until fairly large
  • Diagnosed at advanced stages
  • Symptoms can be anemia and vomiting
  • Higher frequency of: Mucinous tumors, BRAF-mt or KRAS-mt mutations, MSI-H

Cancer on the Left

More often diagnosed in men

Splenic Flexure, Descending and Sigmoid Colon, Rectosigmoid and Rectum

  • Tumors grow along the inside wall narrowing the colon
  • Diagnosed at earlier stages due to symptoms
  • Common symptoms include a change in bowel habits, blood in the stool, and bowel obstruction
  • Higher frequency of HER2 amplifications
  • Overall better prognosis than right-sided tumors

Right-sided Tumor

A right-sided tumor, when diagnosed in early stages (stage I or II), predicts slightly better prognosis than left-sided tumors. Patients with late-stage, right-sided tumors sometimes have KRAS, NRAS, and BRAF mutations. Those mutations can predict poor response to anti-EGFR therapies and shorter survival. Even if right-sided tumors are KRAS, NRAS, or BRAF wild-type tumors, they are unlikely to respond to anti-EGFR therapies, and until recently, survival was typically worse than patients with left-sided tumors. The good news is that recent studies show that right-sided tumors are more often diagnosed as MSI-H, which predicts a positive response to checkpoint inhibitors and immunotherapy treatments.

Left-sided Tumor

A left-sided tumor generally predicts that patients will respond better to anti-EGFR therapies (cetuximab and panitumumab) than to anti-VEGF therapies (bevacizumab) when given with chemotherapy as first-line treatment. A left-sided tumor also predicts better survival for stage IV patients. Prior to starting anti-EGFR agents, you need to have biomarker testing for NRAS, KRAS, BRAF and HER2 because these mutations can also occur in left-sided tumors.

HER2 amplifications are more common in left-sided and rectal tumors, and also predicts a poor response to anti-EGFR agents.

Side Matters: Left vs. Right graphic

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