Cancer after bariatric surgery : risk and prognosis

Obesity is a major public health challenge with a rapidly growing prevalence worldwide. The condition increases the risk of multiple diseases, including cancer in different locations. Current evidence on the effect of intentional weight loss on cancer is inconclusive. Bariatric surgery results in extensive and sustained weight loss with positive impact on several obesity-related comorbidities. It can therefore serve as a proxy for intentional weight. The few studies available suggest that bariatric surgery may decrease overall cancer risk but increase the risk of cancer in specific tumor locations. This thesis assesses the impact of bariatric surgery on cancer risk and cancer mortality using the Nordic Obesity Surgery Cohort (NordOSCo). The cohort includes individuals with an obesity diagnosis recorded in the National Patient Registries in any of the five Nordic countries between 1980 and 2012. Study I and IV included Swedish cohort members only, while Study II and III included the entire cohort. Study I validated bariatric surgery codes in the Swedish Patient Registry and the Scandinavian Obesity Surgery Registry (SOReg) against medical records for bariatric surgery performed during 2011. The accuracy of the codes proved to be high; 93.5% in the Patient Registry and 98.6% in SOReg. However, one fifth of the procedures in SOReg were not registered in the Patient Registry, indicating that the completeness of bariatric surgery registrations in the Patient Registry can be improved. Study II evaluated the association between bariatric surgery and the risk of obesityrelated cancer, non-obesity related cancer and cancer in selected sites. The study found an overall decreased risk of cancer following bariatric surgery that was more evident for cancers associated with obesity, particularly breast cancer, endometrial cancer and nonHodgkin lymphoma in women. The lower cancer risk was primarily observed during the initial years following surgery and this decrease diminished with time after surgery. In contrast, the risk of kidney cancer was increased among bariatric surgery patients. Study III and IV focused on colorectal cancer and studied the impact of bariatric surgery on the risk and prognosis of this disease. Incidence rates of colon cancer were higher in bariatric surgery patients than in the general population, especially after ≥10 years post-surgery. The elevated incidence rates exceeded that of obese individuals without bariatric surgery. No associations were observed for rectal cancer. On the contrary, cancer-specific mortality after rectal cancer diagnosis was more than three-fold increased among bariatric surgery patients compared to obese individuals without this surgery, but no differences in survival were observed between the exposure groups following colon cancer diagnosis. In conclusion, the Swedish Patient Registry is a valid data source for research on bariatric surgery. Overall cancer incidence seems to decrease after bariatric surgery, but the effect varies between cancer sites with a possible increase in the risk of colon and kidney cancer. Bariatric surgery may also decrease the chance of survival in rectal cancer