The risk of colorectal cancer was significantly increased among people who had undergone obesity surgery in a retrospective cohort study of more than 77,000 obese patients enrolled in a Swedish registry.
The increased risk for colorectal cancer was associated with all three bariatric procedures – vertical banded gastroplasty, adjustable gastric banding, and Roux-en-Y gastric bypass – and increased further over time, reported Dr. Maryam Derogar, of the Karolinska Institutet, Stockholm, and her associates. No such pattern over time was seen among the obese patients who did not have surgery.
"Our data suggest that increased colorectal cancer risk may be a long-term consequence of such surgery," they concluded. If the association is confirmed, they added, "it should stimulate research addressing colonoscopic evaluation of the incidence of colorectal adenomatous polyps after obesity surgery with a view to defining an optimum colonoscopy surveillance strategy for the increasing number of patients who undergo obesity surgery. The study was published online in the Annals of Surgery (2013 [doi:10.1097/SLA.0b013e318288463a]).
To address their "unexpected" finding in an earlier study of an apparent increase in the risk of colorectal cancer after obesity surgery, but no increase in the risk of other cancers related to obesity, they conducted a retrospective cohort study using national registry data between 1980 and 2009, of 15,095 obese patients who had undergone obesity surgery and 62,016 patients who had been diagnosed with obesity but did not undergo surgery. They calculated the colorectal cancer risk using the standardized incidence ratio (SIR), the observed number of cases divided by the number of expected cases in that group.
Over a median of 10 years, there were 70 colorectal cancers in the obesity surgery group; and over a median of 7 years, 373 among those who had no surgery. The SIR for colorectal cancer among those who had surgery was 1.60, which was statistically significant. Among those who had no surgery, there was a small, insignificant increase in risk group (a SIR of 1.26). In the surgery group, the risk increased over time in men and women, up to a twofold increased risk among those patients followed for at least 10 years, a pattern than was not observed in the obese patients who had no surgery.
The "substantial increase in colorectal cancer risk, above that associated with excess body weight alone, more than 10 years after surgery is compatible with the long natural history of colorectal carcinogenesis from normal mucosa to a malignant colorectal cancer," the authors wrote. Why the risk was increased is not clear, but one possible explanation could be that the malabsorption effects of the gastric bypass procedure results in local mucosal changes, the authors speculated. Previously, they had identified rectal mucosal hyperproliferation in patients who had undergone obesity surgery, present at least 3 years after the procedure, a finding that was "associated with increased mucosal expression of the protumorigenic cytokine macrophage migration inhibitory factor," they wrote.
The study’s strengths included the size of the sample, long follow-up, and the validity of Swedish national registry data, while the limitations included the retrospective design and the lack of data on body weight over time.
As in the United States and other countries, obesity has been increasing in Sweden, with a corresponding increase in bariatric surgery. Over the last 20 years, the prevalence of obesity in Sweden has doubled, and the annual number of obesity operations performed has increased from 1,500 in 2006 to almost 4,000 in 2009, according to the authors.
The study was supported by the Swedish Research Council. The authors had no conflicts of interest to declare.