Isabelle N. Colmers, MSc, from the University of Alberta in Edmonton, Canada, and colleagues used linked administrative databases (1996 to 2006) to create a cohort of 185,100 adults from British Columbia, Canada, with incident type 2 diabetes, who were age-, sex-, and index date-matched to individuals without diabetes in a 1:1 ratio. During annual time windows following the index date, the incidence rates and hazard ratios for bladder cancer were calculated.
During a median follow-up of four years, the researchers found that within the study population (54% men; average age, 60.7 years) 1,171 new bladder cancer cases were diagnosed. Bladder cancer incidence in the diabetes cohort was 85.3 per 100,000 person-years in the first year after diabetes diagnosis, compared with 66.1 in the control cohort, which translated into a significant 30% increased bladder cancer risk in adjusted analyses. The increased bladder cancer risk seen in the first year was limited to those with the fewest physician visits two years prior to the index date. Type 2 diabetes was not associated with bladder cancer after the first year.
“The results suggest that early detection bias may account for an overestimation in previously reported increased risks of bladder cancer associated with type 2 diabetes,” the authors wrote.