The sigmoid colon is the part of the large intestine (colon) closest to the rectum. It absorbs water from the stool and pushes it into the rectum and anus until it is ready to be expelled when you use the toilet.
Colorectal cancer is a general term for colon cancer and rectal cancer. Studies have shown that colorectal cancer is more common in the sigmoid colon, which is on the left side, along with the descending colon and rectum. The rectum is also considered to be on the left side, although it is not part of the colon.
Gastrointestinal oncologist Dr. Kanwar Raghavestimates that approximately 70% of colorectal cancers are diagnosed on the left side.
“We don’t know why colon cancer occurs in certain locations, although there are several explanations,” says Raghav. “However, left-sided colon cancer is more common than right-sided colon cancer.”
The colon begins at the cecum on the right side. It continues to the ascending colon, crosses the body to the transverse colon, and descends to the left descending colon and sigmoid colon.
We spoke to Raghav to learn more about the differences between left-sided and right-sided colon cancer. Here’s what he shared:
It is common for left-sided tumors to cause symptoms.
Patients with left-sided colorectal cancer often have altered bowel habits and bloody stools. People with tumors on the right side often have fewer symptoms.
The left-sided tumor and the right-sided tumor have different genetic characteristics.
Tumors that occur on the left side of the colon often contain mutations in tumor suppressor genes such as APC and TP53. These genes help prevent tumor growth. Mutations in these genes increase the chance of developing certain cancers, such as colon cancer.
People with hereditary cancer syndromes are more likely to develop tumors on the right side of the colon. These include microsatellite instability-high (MSI-high), also known as DNA mismatch repair deficiency (dMMR), and Lynch syndrome.
Tumors on the left side are more likely to be detected during colonoscopy.
During a colonoscopy, your doctor inserts a flexible endoscope around your rectum and large intestine to examine the walls and lining of your colon. The doctor will examine the left side first, near where the scope will be inserted. Colorectal cancer usually begins as polyps in the colon or rectum.
Tumors on the left side often appear as polypoid lesions, which are easier to see because they protrude into the lumen of the gastrointestinal tract. Colonoscopies are excellent at detecting these polyps, so it is very important to have regular screening tests.
Many polyps on the right side of the colon are sessile serrated adenomas. These are flat and difficult to detect during a colonoscopy. For this reason, right-sided colon cancer is often diagnosed at an advanced stage and can be difficult to treat.
Preparing for a colonoscopy is important because the colon must be completely cleared of stool so that the doctor can easily see both the left and right sides of the colon.
The side where the cancer occurs helps determine treatment.
Overall, left-sided tumors have a better prognosis than right-sided tumors. This is because left-sided colon cancer responds better to chemotherapy and targeted therapies such as anti-EGFR and HER2 drugs.
Right-sided tumors tend to have more genetic mutations and are often associated with more aggressive cancers. Tumors on the right side are also difficult to detect and are often diagnosed later in the disease. Nowadays, immunotherapy is effective in treating these cancers because it allows the body to recognize and attack abnormal cells. Immunotherapy has helped improve the prognosis for patients with stage IV right-sided colon cancer.
Your doctor and care team will work together to create the best treatment plan for you.
Knowledge about left-sided and right-sided colon cancer can lead to advances in treatment.
Our hospital has been treating colorectal cancer for many years. But it’s only in the last four to five years that we’ve really started to understand the differences between colon cancer that occurs on the left side and colon cancer that occurs on the right side. Until then, it was not so commonly recognized.
Over the years, we have learned that colon cancer cannot be considered a single disease. We are now looking at this disease as a complex disease with many subtypes and are trying to develop targeted and individualized therapies that can be used depending on the specific type of person. Slowly and steadily, we are raising the bar for personalized treatment of colorectal cancer. And I think the location of the cancer (left side or right side) is also part of it.
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