Cancers that develop in the colon are not all the same. Where they originate in the colon has important survival and treatment implications.
Geography of the Bowel
At 5-feet long, the colon makes up the bulk of the large intestine, which is also called the bowel. The colon removes nutrients from the food you eat and eliminates solid waste. The rectum and anal canal constitute the last part of the large intestine. Together, they are about 6 to 8 inches long. The colon begins on the right side of your body (the proximal colon) at the end of the small intestine and travels upwards toward your head. It crosses across your body (the transverse colon) and then descends downwards toward the rectum. This is the distal, or descending, colon.
Embryologically, these sections of the colon are different, says Dr. John Mansour, a surgical oncologist at the UT Southwestern Medical Center’s Simmons Comprehensive Cancer Center. In a fetus, the right colon arises from the mid gut, and the left colon arises from the hind gut. They fuse together at the transverse colon during development. Although visually the colon appears as one continuous tube, colorectal cancer tumors don’t just develop in different spots in the same tube. We don’t understand all the reasons why side is important in where colon cancers develop, Mansour says.
[See: 8 Questions to Ask Your Doctor About Colon Cancer.]
The Difference between Tumors on the Left and Right Side of the Colon
“Colorectal cancer is not a [single] disease,” says Dr. Dale R. Shepard, a medical oncologist at the Cleveland Clinic. “[If you look at the] proteins and pathways, there are clear differences in the pathology of tumors on the left and right side.”
As Mansour says: “Initially, doctors noticed right-sided tumors were more likely to present at a later [more advanced] stage. We thought it was because they were further upstream. That may be part of it, but there are real differences [in the tumors as well]. In 2001, [we began to notice] differences in survival. People with left-sided tumors live longer.”
In fact, according to a large, phase 3 clinical trial, colorectal cancer patients with left-sided tumors lived more than a year longer after initial treatment than patients with right-sided tumors.
Eventually, doctors learned that the side where the tumor originated also made a difference in treatment outcomes. CRC patients who have the normal version of a gene called KRAS wild-type (which is known to predict response to certain therapies) responded well when treated with cetuximab. Cetuximab is a type of targeted therapy normally prescribed for people with metastatic colorectal cancer. Mansour says a study showed that people with normal KRAS wild-type had an overall survival rate that was significantly better in left-sided cancers.
[See: 10 Seemingly Innocent Symptoms You Shouldn’t Ignore.]
However, Mansour says, if CRC patients had the mutated version of the KRAS wild-type gene, cetuximab did not help them. “These patients did better with bevacizumab [a different targeted therapy].”
Mansour says a 2016 randomized trial looked at patients with only metastasized KRAS wild-type tumors (the normal version) and found that overall survival in patients with left-sided tumors was much better (36 months) compared to those with right sided tumors (16 months). “The [overall survival] was different, even though the cancer had spread,” he says. “This is really important.”
Tumor sidedness affects certain populations more than others: African-Americans, who tend to develop CRC at younger ages and to be diagnosed at more advanced stages, are more likely to have right-sided tumors, Shepard says. And people with Lynch syndrome, a genetic disorder that significantly raises the risk of colon cancer, are also likely to have right-sided tumors.
It’s not because the tumor starts on the right or left side, Mansour says: “Sidedness is a surrogate for genetic differences in tumors.” Future CRC research will continue to shed light on these differences, which will help drive the development of better treatment options. The future of colon cancer care is increasingly going to depend upon the unique characteristics of each individual’s cancer, and treatment will be much more sophisticated.
So far, it appears sidedness does not affect whether or not primary CRC will recur after treatment. However, when colorectal cancer does recur, right-sided tumors still have a worse prognosis or are more aggressive than left-sided tumors.
[See: 10 Ways to Prepare for Surgery.]
If you’re a patient with CRC, ask your doctor about optimal therapies for your specific type of cancer. Shepard says many of the findings regarding the sidedness of colon cancer tumors came from looking back at other studies. Most doctors want to see primary studies that specifically address the topic of sidedness before they adopt treatment changes. For now, it’s another consideration in overall patient treatment decisions.
More from U.S. News
8 Questions to Ask Your Doctor About Colon Cancer
6 Options for People Who Don’t Want a Colonoscopy
10 Weird Things That Can Make You Poop
Is Your Colorectal Cancer Tumor on the Right or Left Side of Your Body? originally appeared on usnews.com
