Occupational disparities in survival from common cancers in Japan: Analysis of Kanagawa cancer registry

BackgroundLittle is known about occupational disparities in survival for common cancer sites in Japan.MethodsUsing data from a population-based cancer registry, we identified 32,870 cancer patients diagnosed during 1992–2011. We followed the patients for 5 years (median follow-up time 5.0 years). For each individual, we classified their longest-held occupation into 5 classes (upper non-manual, lower non-manual, manual, farmer, and others) following the Erikson-Goldthorpe-Portocarero scheme. Poisson regression models were used to estimate overall and site-specific mortality rate ratios (MRRs) and 95% confidence intervals (CI) for each occupational class, adjusted for sex, age, and diagnosis year. Upper non-manual workers served as the reference group. Additionally, using a binary categorization of occupations (manual workers versus non-manual workers), a causal mediation analysis with 4-way decomposition was performed to investigate the potential mediation of the association between occupation and overall mortality by cancer stage.ResultsOverall prognosis was good in this population (5-year overall survival was 81.7%). Compared with upper non-manual workers, both overall and cancer-specific mortality was higher in lower non-manual workers (MRR=1.14, 95% CI 1.05–1.24) and manual workers (MRR=1.40, 95% CI 1.29–1.53). After adjusting for the mediating influence of prognostic factors (stage and treatment), the observed occupational differences were attenuated but remained significant in manual workers: MRR = 1.23 (95% CI 1.08–1.39). Observed occupational disparities tended to be attributable to common cancers, i.e., stomach and lung among men and female breast cancer. Additionally, manual workers had 1.25 times higher odds for advanced stage. In the mediation analysis, the overall proportion explained by mediating effect of cancer stage was 29% (4% due to mediated interaction and 25% due to pure indirect effect).ConclusionWe documented occupational disparities in survival from commonly-occurring cancers in Japan. Occupational differences in cancer stage may explain one-third of the survival disparities.