Know Your Biomarker: Tumor Sidedness - What's the difference?

When we talk about colorectal cancer, most people think of it as a single disease. But where a tumor starts, on the right side or the left side of the colon, can make a meaningful difference in how it develops, how it’s detected, and who is most at risk. Understanding these differences can empower patients, caregivers, and advocates to recognize symptoms earlier and ask the right questions during medical visits.

This post is the first in a series focused on Know Your Biomarker: Tumor Sidedness, and in it we'll cover what's different about who is affected by right- and left-sided colorectal cancers, the symptoms they cause, and when they're diagnosed.

Colorectal cancer can develop in any part of the colon and rectum.

Right-Sided Colon Cancer

Right-sided colon cancers develop in the cecum, ascending colon, or the first two-thirds of the transverse colon, on the body’s right side. Although less common than left-sided cancers, right-sided tumors tend to present unique challenges.

Who is more affected?
Right-sided colon cancer is more frequently diagnosed in women and in older adults.

How does it appear?
These tumors are often flat-shaped, which means they can grow quietly and harder to find during screening colonoscopy when small. Because they don’t protrude as clearly as polyps, they’re more likely to go unnoticed and without symptoms until they become quite large.

Common symptoms
The right side of the colon develops flatter tumors and has looser stool, so blockages are less common. Instead, symptoms often involve:

• Anemia (due to slow, unnoticed bleeding)
• Fatigue
• Occult (hidden) bleeding

Unfortunately, because these symptoms can be subtle or attributed to other conditions, right-sided cancers are often diagnosed at a later stage.

Left-Sided Colorectal Cancer

Left-sided colon cancers form in the last third of the transverse colon, the descending colon, sigmoid colon,or rectum, on the body's left side. These cancers occur more frequently than right-sided ones and often present more noticeable early signs.

Who is more affected?
Left-sided colon cancer is more common in men and tends to appear in younger patients compared to right-sided disease.

How does it appear?
Left-sided tumors are typically polyp-shaped, protruding into the colon’s space. Because they change the shape or movement of stool more quickly, they are often discovered earlier.

Common symptoms
Changes in bowel function are the hallmark signs, including:

• Constipation
• Changes in bowel habits
• Visible blood in the stool
• Abdominal pain

As a result, left-sided cancers are usually diagnosed earlier, which can improve treatment outcomes.

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For more information about tumor sidedness, please visit our Know Your Biomarker Tumor Sidedness page and check out the next blog post in the series.

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