Background Diabetes is a chronic disease which affects c. 5% of Scottish men; lung cancer is the second most common cancer among men in Scotland. While both conditions exhibit marked social patterning (specifically, higher prevalence in deprived groups), little research has sought to determine whether the relationship between socio-economic status (SES) and lung cancer differs between men with and without diabetes.
Methods Data from a population-based national diabetes register and the Scottish Cancer Registry, combined with all-Scotland population totals, were used to investigate associations between Type 2 diabetes (T2DM), SES and lung cancer. SES was represented by quintiles of the Scottish Index of Multiple Deprivation (a composite area-based measure of multiple deprivation). Age-standardised incidence rates for lung cancer at ages 55–79 during the period 2001–2007 were calculated, by quintile of SES, for men with and without T2DM. Generalised linear models (GLMs) were fitted to estimate the relative risk (RR) of lung cancer associated with T2DM in each SES quintile, adjusted for age.
Results Among men without T2DM, age-standardised incidence of lung cancer (per 100,000 of population) in the least deprived and most deprived quintiles was, respectively, 165.2 (95% CI 155.7, 174.8) and 551.0 (533.4, 568.7). Corresponding rates in men with T2DM were 208.1 (172.4, 243.8) and 363.3 (322.7, 403.8). Relative risk of lung cancer (T2DM vs. no diabetes) ranged from 1.22 (0.98, 1.53) in the least deprived quintile to 0.67 (0.59, 0.76) in the most deprived quintile, decreasing monotonically in intermediate quintiles. Across all SES quintiles combined RR of lung cancer in men with T2DM was 0.81 (0.73, 0.89). The interaction of SES with T2DM was highly significant (p < 0.001).
Conclusion Marked social gradients for lung cancer incidence were observed both in men with and without T2DM. However, the influence of SES on cancer incidence differed materially by diabetes status: among more deprived men, the risk of lung cancer was attenuated in those with Type 2 diabetes. Possible explanations include lower prevalence of obesity and risk of diabetes among deprived men who smoke, higher prevalence of diabetes among this group, survival bias and more intensive smoking cessation guidance given to deprived men with diabetes in the course of their ongoing clinical management.
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