Cancer in the sigmoid colon: What it means when colon cancer is on the left side

The sigmoid colon is the part of the large intestine (colon) closest to the rectum. It absorbs water from stool and pushes the stool to the rectum and anus until it’s ready to be expelled when you use the bathroom.

Colorectal cancer is an umbrella 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 the left side even though it’s not part of the colon.

Gastrointestinal medical oncologist Kanwal Raghav, M.D., estimates that about 70% of colorectal cancers are diagnosed on the left side.

“It’s unclear why colon cancer starts in a particular location, although some explanations exist,” says Raghav. “However, left-side colon cancers are more common than right-side colon cancers.”

The colon starts at the cecum, which is on the right side. It goes up to the ascending colon, across the body to the transverse colon and down to the descending colon and sigmoid colon, which are on the left side.

We spoke with Raghav to learn more about the differences between left-side and right-side colon cancer. Here’s what he shared.

It’s common for left-side tumors to cause symptoms.

It's common for people with left-side colorectal cancer to have changes in their bowel habits or blood in their stool. People with right-side tumors often show fewer symptoms.

Left-side tumors and right-side tumors have different genetic characteristics.

Tumors that develop on the left side of the colon have more mutations of tumor-suppressing genes, like APC and TP53. These genes help prevent tumors from growing. Mutations in these genes make you more likely to develop certain cancers, including colon cancer.  

Tumors that develop on the right side of the colon are more likely in people who have hereditary cancer syndromes. These include high microsatellite instability (MSI-high) – also referred to as DNA mismatch repair deficiency (dMMR) – and Lynch syndrome.

Left-side tumors are easier to detect during a colonoscopy.

During a colonoscopy, your doctor examines the walls and lining of your colon by inserting a flexible scope into your rectum and around your large intestine. Doctors examine the left side first since it’s closer to where the scope is inserted. Colorectal cancer usually starts as a polyp in the colon or rectum.

Left-side tumors often present as polypoid lesions, which protrude into the lumen of the gastrointestinal tract and are easier to see. Colonoscopies are good at detecting these polyps, which is why getting this routine screening is so important.

Many polyps on the right side of the colon are sessile serrated adenomas. These are flat and harder to detect during a colonoscopy. Because of this, right-side colon cancers are often diagnosed at advanced stages and may be harder to treat.

Colonoscopy prep is important because your colon will need to be completely clean of any stool, so your doctor can easily see both the left and right side of your colon.

The side on which cancer develops helps determine your treatment.

Overall, left-side tumors have a better prognosis than right-side tumors. This is because colon cancer on the left side responds better to chemotherapy and targeted therapies, like anti-EGFR and HER2 drugs.

Right-side tumors tend to have more genetic mutations, which are often associated with more aggressive cancers. Right-side tumors are also harder to detect, so they’re often diagnosed when the disease is more advanced. Recently, immunotherapy has been effective in treating these cancers because the body can recognize the abnormal cells and attack them. Immunotherapy has helped improve the prognosis for patients with stage IV right-side colon cancer.

Your doctor and care team will work together to develop the best treatment plan for you. 

Knowledge of left-side and right-side colon cancer leads to treatment advances.

We’ve been treating colorectal cancer for a long time. But only in the last four or five years have we really started to understand the differences between colon cancer that develops on the left side versus the right side. Before then, it wasn’t as commonly acknowledged.

Over the years, we have learned that colon cancer can’t be viewed as one disease. We’re now looking at it as a complex disease with many subtypes and trying to develop targeted and personalized therapies that can be used depending on the specific type people have. Slowly and steadily, we are moving the bar for personalized therapy in colorectal cancer. And I think the location of the cancer – left side versus right side – is a part of that. 

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