Unknown primary cancer in NSW

Introduction. Carcinomas of unknown primary are characterised as cancers that are disseminated early with no apparent presence of a primary tumour and with an unpredictable pattern of metastasis. Methods: NSW Central Cancer Registry data were used in an analysis of:(I) all cancers diagnosed from 1972 to 2003 to analyse time trends; (2) cases diagnosed in 1999-2003 in relation to predictor characteristics; and (3) cases diagnosed between 1980 and 1995, and followed to the end of 2001, for survival assessment. SAS (Statistical Analysis Software) was used for univariate and multivariate logistic regression to explore the association with the outcome variable of unknown primary versus known primary for each of the potential explanatory variables of sex, age, socioeconomic status and ARIA (index of remoteness). Cause-specific survival analysis was undertaken using Kaplan Meier and multivariate modelling was completed using Cox proportional hazards regression.

Results: There was little difference between the pattern of presentation of unknown stage in male and females over time. Fluctuation in the incidence of CUP have occurred since 1972, however the impact of changing diagnostic and clinical criteria is uncertain. The majority of unknown primary cancers are late stage (distant metastases). Those diagnosed with distant metastases have an 80% (95%CI: 70% to 92%) greater chance of dying in the first year after diagnosis compared to cancers of the same stage where the primary site is known. Those with an unknown primary cancer are more likely to be female, aged over 75 years and less likely to be histologically verified at diagnosis. Among metastatic cases, the risk of death for unknown primaries is 23% (95% CI 19-26'1'0) higher than for known primaries, when controlling for sex, age, histology and socioeconomic status.

Conclusion : Metastatic unknown primary cancer behaves differently to metastatic known primary cancer in terms of, explanatory variables and survival outcomes, supporting the view that unknown primary cancer is an entity in its own right not just the absence of a primary diagnosis.