Obesity Surgery and the Risk of Colorectal Carcinoma—Searching for the Fly in the Ointment?

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To the Editor:
We would like to congratulate Derogar et al1 for publishing their study “Increased Risk of Colorectal Cancer after Obesity Surgery” in the distinguished journal Annals of Surgery. Derogar et al1 suggested that patients after obesity surgery had a higher risk to develop colorectal cancer (CRC) over time when compared with an obese control group. The authors concluded that colonoscopy surveillance for patients who underwent obesity surgery may be required. The strengths of the study include the large sample size, the length of follow-up, and the use of high quality data of the Swedish National Registers.
Obesity surgery is currently the only weight loss therapy with a proven survival benefit,2,3 but surgery as a treatment of morbid obesity is still questioned by many. Therefore, data suggesting negative effects of obesity surgery, such as an increased CRC risk, are grist to the mills of skeptics.
Although the authors have an excellent track record in epidemiology, we have concerns regarding the conclusions of Derogar et al1 due to several methodological problems including the following:
First, the CRC risk in the obese nonsurgery group did not increase over time as would be expected.4 The authors report a high percentage of obesity-associated comorbidities such as diabetes, hypertension, or cardiovascular diseases (23%, 30%, and 32%, respectively) for the nonsurgery group but fail to provide mortality data. The possibility that the nonsurgery group died before they developed CRC is not addressed, neither is whether patients of the obesity surgery group lived longer because their metabolic conditions were effectively treated.2,3 If patients live longer because of a reduction in cardiovascular or metabolic diseases, it may be possible that they have longer time to develop CRC. However, the authors failed to address this in their article.
Second, it is surprising that there was no difference in the CRC risk between the 3 types of surgery [vertical banded gastroplasty, gastric banding, and roux-en-y gastric bypass (RYGB)], which have very different anatomy and underlying physiological mechanisms.5,6 However, looking at the CRC incidence during the observation period, substantial differences exist: 31 of 3743 vertical banded gastroplasty patients (0.83%), 27 of 3575 gastric banding patients (0.75%), and 12 of 7769 RYGB patients (0.15%). The authors suggest that bile flow, bile metabolism, or alterations in gut microbiota may play a role in the development of CRC, but the authors fail to explain why patients after RYGB (the only surgery where bile flow is altered) did not have a higher CRC incidence. The study appeared underpowered to examine this hypothesis.
Third, any comparison between the obese surgery group and the obese nonsurgery group is problematic as body weights were not reported. Subjects were included if they had a diagnosis of obesity at admission to any hospital, but no information on either the severity of obesity or reasons for hospital admission is provided. The authors stated that they “never intended to formally compare” these 2 cohorts directly as it would be “hazardous” but then go on to conclude that “The absolute cumulative incidence of colorectal cancer in the obesity surgery cohort was lower (48 per 100,000 person years) than that of the obese no surgery cohort (91 per 100,000 person years).

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