Colorectal Cancer Incidence Is Falling in Western Countries – Why Does Screening Not Explain Everything?

Unlike breast and prostate cancer screening for early diagnosis, screening for large bowel (colon and rectal) cancer not only enables early diagnosis but also prevents disease development. In the 1960s, Gilbertsen first suggested that polypectomy (polyp removal) could prevent colorectal cancer formation. Some 20 years later, Vogelstein described the transition from polyp to cancer as a stepwise process and detailed genetic changes at each step. Most colorectal cancers are known to develop from polyps over a long period, providing sufficient time for both early diagnosis and prevention. This suggested screening was ideal for reducing mortality.

“There have been 9 large clinical studies on the effect of imaging on both colorectal cancer incidence and mortality. Screening methods such as sigmoidoscopy (examining only the final large bowel sections for cancer using an endoscope with a small camera at its tip) and colonoscopy (imaging the entire large bowel with an endoscope) were observed to reduce colorectal cancer mortality by 15% to 30%. Early diagnosis through this was suggested as primarily responsible for the mortality reduction. Meanwhile, fecal occult blood testing produced a 5% fall in colorectal cancer mortality.

Against this backdrop, considering colorectal cancer and mortality trends in American adults aged 50 and over, the big picture is clearly good news. Since 1975, overall colorectal cancer rates have fallen approximately 40%, and since peaking in the mid-1980s, they have fallen more than 45%. More importantly, colorectal cancer mortality has fallen by more than half.

These downward trends have been attributed to wider screening. However, other factors should be considered in these large changes. No study of screening for colorectal or any other cancer has shown a 50% reduction in mortality.

More importantly, the timing of trends does not match this explanation. Population-wide colorectal screening spread slowly. The National Health Interview Survey reported that in 1987, among Americans aged 50 and over, only 23% had been screened for colorectal cancers. Approximately 20 years later, in 2005, this had risen only to 50%. Given these slow screening increases and expected lag in mortality effects, understanding population screening effects before several decades pass is difficult.

If not screening, what explains falling colorectal cancer mortality? We believe plausible explanations fall into 3 categories:

• First, current colorectal cancer treatment is much better than 30 years ago. Improved surgical techniques, standardized care before and after surgery and high confidence in physicians influence declining mortality. Adding adjuvant chemotherapy for patients with regional (node-positive) cancer has also reduced long-term mortality. Even patients with widely spread cancer undergo surgery for distant metastases, with survival extended by 5 years or more in a quarter of these patients.
• Second, in symptomatic disease (with complaints, signs or findings), early diagnosis and subsequent mortality reduction have appeared even without broad screening. Colorectal cancer symptoms generally appear sooner and diagnosis is made earlier than previously. Increases in early-stage (local) and regionally advanced disease in the late 1970s, early 1980s and again late 1990s reflect growing sigmoidoscopy and colonoscopy use. Falling metastatic disease rates—the proportion with metastatic colorectal cancer—may be explained by earlier diagnosis of these aggressive cancers and have been an important step in reducing mortality.
• Third and finally, colorectal cancer cases may have decreased. Metastatic disease rates have fallen steadily and substantially since 1975 (45%). The decline in overall rates began before the expected effect of polypectomy. Falling rates before more frequent diagnosis would reliably lead to lower mortality.

Several factors may account for declining colorectal cancer rates. A clear candidate for gastrointestinal organ cancers is dietary change. Reduced consumption of salted and smoked meat reduces exposure to carcinogenic nitrosamines. Changes in gastrointestinal microbiomes (beneficial bacteria known as gut flora) are another clear candidate. Widespread antibiotic use may have changed gut flora (as demonstrated by the reduction in previously common Helicobacter pylori). Finally, use of nonsteroidal anti-inflammatory drugs (including aspirin), hormone replacement therapy and statins (lipid-lowering drugs) has played a role in declining colorectal cancers.

None of this should suggest that screening has no effect on declining colorectal cancer mortality. Rapid falls in both early-stage and regionally advanced disease in recent years may be attributed to protective effects of increased polypectomy. Colonoscopy rates doubling between 2000 – 2005—in American adults aged 50 and over, from 20% - 39%—may explain this trend.

Nevertheless, we believe physicians should show some humility about screening’s effect on disease trends. Although taking credit for good news is appealing, doing so may exaggerate perceived benefits of general population screening and distract from more important health-promoting activities—for example, encouraging healthy eating and exercise. Most screened people have neither had cancer detected nor been protected against its development, and they undergo regularly repeated colonoscopies to check small polyps. Benefits and harms must be carefully weighed when referring very old or too frail people to benefit from screening colonoscopy. Stomach cancer offers a striking example in questioning the argument that screening strongly explains declining colorectal cancer incidence and mortality: since 1930, stomach cancer incidence and mortality have fallen approximately 90% without screening efforts…”

The article shared above was published in NEJM (the New England Journal of Medicine), one of the most respected and influential scientific journals, on 4 May 2016. We wanted to share it because it offers valuable information and different perspectives. Although colorectal cancer rates appear to decline in Western countries, we must state this is not yet the case in our country. According to our 2015 National Cancer Statistics, colorectal cancers are the 3rd most common cancer type in both men and women. A slight increase in colorectal cancer incidence among young people is also observed. With major advances and wider availability of healthcare in our country, cancer screening programs carried out with great effort by our Ministry of Health through KETEM centers, and public awareness efforts to which I believe we contribute, cancer prevention and early diagnosis will become more widespread in our country and cancer mortality will consequently decrease.