Exploring the epidemiological characteristics of cancers of unknown primary site in an Australian population: Implications for research and clinical care

Objectives: To investigate incidence, mortality and case survival trends for cancer of unknown primary site (CUP) and consider clinical implications. Method: South Australian Cancer Registry data were used to calculate age-standardised incidence and mortality rates from 1977 to 2004. Disease-specific survivals, socio-demographic, histological and secular predictors of CUP, compared with cancers of known primary site, and of CUP histological types, using multivariable logistic regression were investigated. Results: Incidence and mortality rates increased approximately 60% between 1977-80 and 1981-84. Rates peaked in 1993-96. Male to female incidence and mortality rate ratios approximated 1.3:1. Incidence and mortality rates increased with age. The odds of unspecified histological type, compared with the more common adenocarcinomas, were higher for males than females, non-metropolitan residents, low socio-economic areas, and for 1977-88 than subsequent diagnostic periods. CUP represented a higher proportion of cancers in Indigenous patients. Case survival was 7% at 10 years from diagnosis. Factors predictive of lower case survival included older age, male sex, Indigenous status, lower socio-economic status, and unspecified histology type. Conclusion: Results point to poor CUP outcomes, but with a modest improvement in survival. The study identifies sociodemographic groups at elevated risk of CUP and of worse treatment outcomes where increased research and clinical attention are required.