Descriptive epidemiology of cancer of unknown primary site in Scotland, 1961-2010

Cancers of unknown primary site (CUP) pose problems for diagnosis, treatment, and accurate prediction of prognosis. However, there are limited published data describing the epidemiology of this disease entity. Our aim was to describe the epidemiology of CUP in Scotland.

Anonymised data, covering the period 1961-2010, were extracted from the Scottish Cancer Registry database, based on the following ICD-10 diagnostic codes: C26.0, C26.8, C26.9, C39, and C76-C80. Age-standardised incidence rates were calculated by direct standardisation to the World Standard Population. Estimates of observed survival were calculated by the Kaplan-Meier method.

Between 1961 and 2010, there were 50,941 registrations of CUP, representing 3.9% of all registrations of invasive cancers. Age-standardised rates increased to a peak in the early to mid-1990s, followed by a steeper decrease in rates. During 2001-2010, age-standardised rates of CUP were higher in the most compared with the least deprived fifth of the population. Observed survival was marginally higher in patients diagnosed during 2001-2010 (median 5.6 weeks) compared with those diagnosed in the previous two decades. During the most recent decade, survival decreased with age at diagnosis, and was higher in patients with squamous cell carcinoma and with lymph node metastases.

Patterns of CUP in Scotland are largely consistent with those reported from the few other countries that have published data. However, in comparing studies, it is important to note that there is heterogeneity in terms of definition of CUP, as well as calendar period of diagnosis or death. Variation in the definition of CUP between different epidemiological studies suggests that there would be merit in seeking international agreement on guidelines for the registration of CUP as well as a standard grouping of diagnostic codes for analysis.