Risk of Obesity-Related Cancer After Obesity Surgery in a Population-Based Cohort Study

Obesity increases the risk of several cancers, but it is uncertain whether weight reduction is followed by any decreased risk. To address this topic, we selected a group of patients representing a substantial weight loss starting at a defined time, ie, patients submitted to obesity surgery. We hypothesized that risk of obesity-related cancer decreases with time after obesity surgery. A nationwide, population-based cohort study of obesity surgery in 1980-2006 as registered in the Swedish Patient Register. New cancers were identified through the Swedish Cancer Register. Cohort members' observed total number of overall obesity-related cancers and groups of obesity-related cancer (breast, prostate, colorectal, endometrial, kidney) were divided by the expected numbers, representing the baseline risk, thus calculating standardized incidence ratios (SIRs) with 95% confidence intervals (CIs). Time trends of SIR after obesity surgery were the main outcome measure. Among a total of 13,123 obesity surgery patients, contributing with 125,049 person-years of follow-up, 296 new cases of obesity-related cancer were identified. There was no overall decrease in SIR of obesity-related cancer with increased time after obesity surgery (P for trend 0.40). Similarly, no statistically significant trends with follow-up time were found for cancer of the breast (P = 0.60), prostate (P = 0.34), endometrium (P = 0.83), or kidney (P = 0.42), while the risk of colorectal cancer increased with time (P for trend 0.01) after obesity surgery. The weight reduction following obesity surgery might not be entailed by a decreased risk of obesity-related cancer with increasing follow-up time as compared with the baseline risk.