How well does colonoscopy prevent colon cancer? What you need to know

Media reports about a new study have sparked largely unfounded controversy.

Colorectal cancer (CRC) is the second leading cause of cancer death among Americans. The gold standard screening test, colonoscopy, is performed on approximately 15 million people in the United States each year. In 2021, due to rising rates of CRC in younger people, the US Preventive Services Task Force lowered the recommended screening age from 50 to 45. For certain patients, including those with a family history of early-onset CRC or diseases such as inflammatory bowel disease, screening may also begin before age 45.

Previous research shows that colonoscopy is associated with a reduction in new cases of colorectal cancer by up to 69% and an 88% reduction in the risk of death. When several news reports described the results of a recent randomized colonoscopy trial as disappointing (see examples here and here), it came as a surprise to many — including some experts like us who carefully combed through the study.

Why is colonoscopy the gold standard for colorectal cancer screening?

As many know, a colonoscopy involves inserting a long, flexible tube with a camera at the end into the rectum. A highly trained gastroenterologist or surgeon guides the tube up the colon and looks for growths called polyps or other abnormalities, including early-stage cancer. Colonoscopy can not only identify potentially precancerous polyps, called adenomas, but also allow them to be removed. A similar (though less comprehensive) screening test called flexible sigmoidoscopy evaluates only the lower colon and may miss cancers further in the colon, which tend to have a worse prognosis.

Other screening tests for CRC cannot locate or remove polyps. They look for blood or abnormal DNA in the stool, which could indicate a polyp or cancer. These include fecal occult blood tests and fecal immunochemical tests, which detect blood, and Cologuard, which detects blood and abnormal DNA.

All these key factors make colonoscopy the preferred choice.

What does this new study tell us about colonoscopy?

Published inThe New England Journal of MedicineThe study was a randomized controlled trial involving 84,585 people between the ages of 55 and 64 in Norway, Poland and Sweden who had not previously undergone screening. Participants received either an invitation to a screening colonoscopy or no invitation. Researchers followed participants for 10 to 15 years to compare the number of colorectal cancers and deaths from CRC in each group.

Only 42% of people who were invited to have a colonoscopy accepted the invitation. The data of all invited people, regardless of whether they actually underwent a colonoscopy, is called “intention-to-screen” analysis. As many news reports rightly noted, the intention-to-screen analysis showed an 18 percent reduction in subsequent colon cancers and no significant reduction in deaths. Importantly, however, when only people who actually had a colonoscopy were analyzed (known as a “per protocol” analysis), colonoscopy reduced colorectal cancer cases by 31% and CRC-related deaths by 50%.

Why are these findings so different?

In this study, a relatively small percentage (42%) of people invited for a colonoscopy actually underwent the procedure, compared to a rate of 60% of adults in the United States, where colonoscopy is widely recommended. This low participation rate is the main reason why the intention-to-screen analysis showed lower detection and death rates than the per-protocol analysis. After all, you won't find anything if you don't look for it.

This is not to say that intention-to-screen analysis is meaningless. It directs our focus to real barriers that lead to the low participation rate. For example:

• Hat die Einladung der Untersucher die Vorteile der Koloskopie angesichts der fehlenden Möglichkeit für ein ausführliches, persönliches Gespräch mit einem Arzt angemessen vermittelt?
• Konnten sich die Menschen für das Verfahren von der Arbeit frei nehmen?
• Fanden einige die Vorbereitung zu entmutigend?

We know that these factors can prevent people from having a recommended colonoscopy.

What else is important to know?

An important limitation of the study is that the detection rate of precancerous polyps by colonoscopy was lower (31%) than usual in the United States (approximately 40%). A lower detection rate may result in fewer polyps being removed, thereby weakening the potential for fewer future deaths from CRC.

After all, cancer doesn't grow overnight. Precancerous lesions can take years to develop into cancer if left unchecked. Therefore, the authors plan to repeat their analysis in 15 years to learn whether the benefits of screening colonoscopy increase over time.

Conclusion: Should you undergo a screening colonoscopy or not?

Yes! Definitely, yes.

The take-home message from this study is that deaths from colorectal cancer are reduced by half when people undergo screening colonoscopy. That's a huge reduction! And while the CRC death rate is often the bottom line of studies, it's important to consider the difficulties associated with a cancer diagnosis - financial costs, physical costs of surgery, chemotherapy and radiation - as well as the suffering caused by the disease itself. This study teaches us that colonoscopies work quite well when they are done and that we still need to work on making colonoscopy more accessible so that more people can benefit from screening.