The currently declining incidence of cancer of unknown primary.

Cancer of unknown primary (CUP) is a heterogeneous group of metastatic cancers for which a primary tumor cannot be identified after a standardized work-up. The biology of CUP has not been fully elucidated and epidemiologic data may be helpful in this regard. The variations in the incidence-rate over time and between countries reflect changes in the risk factors for CUP, incidence trends of the primary tumors that potentially contribute to the burden of CUP and changes in the diagnostic technologies and practice. CUP accounted for 3-5% of cancers in the historical series but its incidence seems to decline in the recent publications. This paper reviews the published cancer-registry studies in order to identify and understand the variations in the incidence-rates of CUP.