Background: The relationship between comorbid disease and health service use and risk of cancer of unknown primary site (CUP)is uncertain.
Methods: A prospective cohort of 266,724 people aged 45 years and over in New South Wales, Australia. Baseline questionnaire data were linked to cancer registration, health service records 4–27 months prior to diagnosis, and mortality data. We compared individuals with incident registry-notified CUP (n = 327; 90% C80)to two sets of randomly selected controls (3:1): (i)incident metastatic cancer of known primary site (n = 977)and (ii)general cohort population (n = 981). We used conditional logistic regression to estimate adjusted odds ratios (ORs)and 95% confidence intervals (CIs).
Results: In fully adjusted models incorporating sociodemographic and lifestyle factors, people with cancer registry-notified CUP were more likely to have fair compared with excellent self-rated overall health (OR 1.78, 95% CI 1.01–3.14)and less likely to self-report anxiety (OR 0.48, 95% CI 0.24−0.97)than those registered with metastatic cancer of known primary. Compared to general cohort population controls, people registered with CUP were more likely to have poor rather than excellent self-rated overall health (OR 6.22, 95% CI 1.35–28.6), less likely to self-report anxiety (OR 0.28, 95% CI 0.12−0.63), and more likely to have a history of diabetes (OR 1.89, 95% CI 1.15–3.10)or cancer (OR 1.62, 95% CI 1.03–2.57). Neither tertiary nor community-based health service use independently predicted CUP risk.
Conclusion: Low self-rated health may be a flag for undiagnosed cancer, and an investigation of its clinical utility in primary care appears warranted.