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Side Matters: Left vs. Right


The colon is part of the digestive system. It is a 6-foot long, tube-like organ that processes and prepares the removal of waste from the body. The bodily structure of the colon and rectum are divided into the right side (consists of the cecum, ascending colon, hepatic flexure, and the transverse colon) and the left side (splenic flexure, descending colon, sigmoid, and rectosigmoid). The rectum is the body part that connects the left side of the colon to the anus.

Recent studies show that stage IV, metastatic tumors on the right side of the colon do not respond in the same way to certain chemotherapy drugs and targeted-therapy drugs as tumors on the left side of the colon and the rectum. Studies also show a difference in survival. Patients with left-sided tumors typically have a better prognosis.

Right-sided Tumor


A right-sided tumor, when diagnosed in early stages (stage I, II), predicts slightly better prognosis than left-sided tumors. Patients with right-sided tumors in later stages (III, IV) are more often diagnosed with biomarkers such as mutant KRAS and mutant BRAF that predict poor response to anti-EGFR therapies and shorter survival. But, even if right-sided tumors have no KRAS, NRAS, or BRAF mutation, they are unlikely to respond to anti-EGFR therapies, and, until recently, their 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 with MSI-H, a biomarker that predicts positive response to immunotherapy treatments with checkpoint inhibitors and immunotherapy treatment. Therefore, the prognosis of patients with right-sided tumors is likely to improve.

Left-sided Tumor


A left-sided tumor generally predicts that patients will respond better to anti-EGFR therapies than to anti-VEGF therapies when they are given together with chemotherapy as first-line treatment. Also, a left-sided tumor predicts better survival for stage IV patients. However, it is still very important to talk to your doctor and get tested for biomarkers because KRAS, NRAS and BRAF mutations that predict poor response to anti-EGFR therapies still happen in left-sided tumors.

In addition, another biomarker that predicts poor response to anti-EGFR therapies, HER2, is more common in left-sided tumors and in rectal tumors.

It is important to test for HER2 before starting treatment with anti-EGFR therapies, especially if no mutations in KRAS, NRAS and BRAF were found.


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