Colorectal Cancer Screening: Not All It's Cracked Up to Be?

Nick Mulcahy

April 27, 2016

"Unambiguous good news" — that's what the trends are in colorectal cancer incidence and mortality for adults 50 years and older in the United States, according to a pair of experts.

Since 1975, incidence has dropped by about 40% and mortality by about 50%, observe Gilbert Welch, MD, MPH, and Douglas Robertson, MD, MPH, from the Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, New Hampshire.

"These trends are often attributed to screening," the pair write in an essay published in the April 28 issue of the New England Journal of Medicine.

But Drs Welch and Robertson are skeptical that screening has been that powerful.

The "magnitude of the changes alone" suggest that "other factors must be involved," they argue.

They also point out that screening is not always needed for a gastrointestinal cancer to decline dramatically in the United States. "Since 1930, without any screening effort, gastric cancer incidence and mortality have decreased by almost 90%," they report.

However, an expert in the field believes the pair has gone too far with their skepticism.

 
Questioning the efficacy and value of colonoscopy-driven, population-based screening may cost lives.
 

"Questioning the efficacy and value of colonoscopy-driven, population-based screening may cost lives," Thomas Weber, MD, professor of surgery at the State University of New York Downstate Medical Center in New York City, told Medscape Medical News.

But the essayists point out that the overcrediting of screening has potentially negative consequences.

If gastroenterologists and other screening clinicians are taking too much "credit" for these disease trends, this could "exaggerate" the benefits of screening, Drs Welch and Robertson write.

This, in turn, could "distract from the more important activities of promoting health — for example, by encouraging a healthful diet and exercise — and caring the sick," they say.

Furthermore, although the "majority" of people who undergo colonoscopy screening have no cancer or large precancerous polyps, "they often endure repeated colonoscopy for surveillance of small polyps," the essayists explain.

Dr Weber, who is president and founder of the Colon Cancer Challenge Foundation, believes that the pair ignored some "important data trends" in colorectal cancer.

Most notably, he believes data indicate that the incidence of colorectal cancer is not actually decreasing at this point.

Although the incidence in adults 50 years and older has decreased, there is an increasing incidence of colorectal cancer in adults younger than 50 years. This population is too young to undergo colonoscopy (it is recommended that it start at age 55 in the United States) and, thus, cannot possibly benefit from the preventive effect of polypectomy. So this younger group better reveals what is actually happening in terms of colorectal cancer, he suggested.

There has been a 51% increase in the incidence of colorectal cancer since 1994 among adults 20 to 49 years of age in the United States, he specifically noted, citing SEER data.

 
It's not necessarily the case that the incidence is decreasing.
 

Dr Weber also claimed that countries that do not have colonoscopy-driven, population-based screening, such as Canada and the United Kingdom, have had a stable or increasing incidence of colorectal cancer. In other words, in countries akin culturally to the United States that do not have a screening program, the cancer is not decreasing.

"I think these data suggest that it's not necessarily the case that the incidence is decreasing [in the United States]," he said.

What the Essayists Say

In their essay, Drs Welch and Robertson assert that the decreases in colorectal cancer incidence and mortality in those older than 50 years cannot be fully explained by screening.

Screening is not that potent, they say, citing a Cochrane review of nine screening trials (four of fecal occult blood testing and five of sigmoidoscopy) (Cochrane Database Syst Rev. 2013;9:CD009259). In the various studies, reductions in mortality ranged from 14% to 28% and in incidence ranged from 5% to 18%.

These numbers from clinical trials fall far short of what is happening in the real world in the United States.

Furthermore, the timing of the mortality and incidence trends in the United States from 1975 to 2010 do not work with the theory that screening is the lone factor producing the dramatic declines, the essayists explain.

There has actually been a slow uptake of colon screening in the United States; only in 2005 did the rate reach 50% of eligible adults. Clinical trials have revealed that there is at least a decade-long delay between screening and reduced colorectal cancer incidence and mortality.

The declines have been steady in the United States since 1975 and do not correlate well with jumps in the screening rate. The trends do not match up, say Drs Welch and Robertson.

So what explains the decrease in mortality?

Three things, they say.

First, treatments have gotten better over time. Second, there is earlier detection of symptomatic cancer (which can reduce mortality even in the absence of screening) because of better awareness and better diagnostics in the clinic. Third, "there could be fewer cases of colorectal cancer in the first place," say Drs Welch and Robertson.

Dr Weber agrees with the pair that two of these phenomena have been at play, but disagrees with the third idea, as noted above.

What explains the decrease in incidence?

Again, the essayists cite three things.

An "obvious candidate" is diet, especially the reduction in the consumption of smoked and cured meats in the United States. A second factor is the increased use of antibiotics, which have reduced the prevalence of deleterious bacterial flora, such as Helicobacter pylori. Third is the increased use of nonsteroidal anti-inflammatory drugs, including aspirin.

Drs Welch and Robertson do not argue that screening has been ineffectual. They know it works to some extent. However, they want "clinicians to have some humility regarding the effect of screening on disease trends."

Dr Welch, Dr Robertson, and Dr Weber have disclosed no relevant financial relationships. The Colon Cancer Challenge Foundation lists among its benefactors AmeriPath/Quest Diagnostics, Clinical Genomics, Ferring Pharmaceuticals, and Polymedco.

N Engl J Med. 2016;374:1605-1607. Abstract

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