Cause-specific mortality in Scottish patients with colorectal cancer with and without type 2 diabetes (2000-2007).

AIMS/HYPOTHESIS: The objective of this study was to use Scottish national data to assess the influence of type 2 diabetes on (1) survival (overall and cause-specific) in multiple time intervals after diagnosis of colorectal cancer and (2) cause of death.
METHODS: Data from the Scottish Cancer Registry were linked to data from a population-based national diabetes register. All people in Scotland diagnosed with non-metastatic cancer of the colon or rectum in 2000-2007 were included. The effect of pre-existing type 2 diabetes on survival over four discrete time intervals (<1, 1-2, 3-5 and >5 years) after cancer diagnosis was assessed by Cox regression. Cumulative incidence functions were calculated representing the respective probabilities of death from the competing causes of colorectal cancer, cardiovascular disease, other cancers and any other cause.
RESULTS: Data were available for 19,505 people with colon or rectal cancer (1,957 with pre-existing diabetes). Cause-specific mortality analyses identified a stronger association between diabetes and cardiovascular disease mortality than that between diabetes and cancer mortality. Beyond 5 years after colon cancer diagnosis, diabetes was associated with a detrimental effect on all-cause mortality after adjustment for age, socioeconomic status and cancer stage (HR [95% CI]: 1.57 [1.19, 2.06] in men; 1.84 [1.36, 2.50] in women). For patients with rectal cancer, diabetes was not associated with differential survival in any time interval. CONCLUSIONS/
INTERPRETATION: Poorer survival observed for colon cancer associated with type 2 diabetes in Scotland may be explained by higher mortality from causes other than cancer.