Comparison of internal and external reference populations for occupational cancer surveillance in a cohort drawn from a diverse workforce

Objectives

Prior analyses of the Occupational Disease Surveillance System (ODSS) have compared cancer rates using internal referent groups. As an exploratory analysis, we sought to estimate cancer risk using general population reference rates to evaluate the impact that the comparison population has on findings from our surveillance program.

Methods

A cohort of approximately 2.3 million workers in Ontario, Canada with an accepted lost-time workers' compensation claim were followed for all cancer diagnoses between 1983 and 2018. Standardized incidence ratios (SIRs) and 95% confidence intervals were calculated for workers in specific occupational groups using (1) all other workers in the ODSS cohort, and (2) the general population of Ontario.

Results

SIRs using the general population reference group were generally equal to or modestly lower compared to SIRs using the internal reference group. Within occupation groups, SIRs had a discordant direction of association (increased rate in the internal comparison and decreased in the external comparison) for some cancer sites including urinary, prostate, and colorectal.

Conclusions

Findings emphasize the importance of the choice of reference group when evaluating cancer risks in large occupational surveillance cohorts. Importantly, the magnitude of confounding and the healthy worker hire bias may depend on the occupation group and cancer site of interest.