Metastasis is a process whereby cancer cells leave the primary site of origin and travel to other parts of the body. If a metastatic cancer is detected, but the primary tumor site cannot be identified, the patient is diagnosed with cancer of unknown primary site (CUP). Approximately 3% to 5% of all reported cancers are diagnosed as CUP. Determining the primary tissue of origin for CUP patients may be of clinical benefit as current treatments for cancer are based largely on the anatomic primary site. Knowing the primary tissue type with greater certainty may help physicians prescribe more effective treatment and improve patient survival. Molecular biology data suggest that tumor cells maintain the basic genetic signature of their tissue of origin, even after metastasis. The development of gene expression profiling technology over the past 10 years has allowed a more molecular approach to diagnosis of the primary tumor site. Gene expression microarrays can measure the expression levels of thousands of genes simultaneously using gene-specific probes
and fluorescent markers. Pathwork Diagnostics Inc. has developed the Pathwork® Tissue of Origin Test to measure gene expression levels of more than 1500 genes in tumor samples of unknown origin, and compare them to the expression profile patterns of a panel of 15 known tissue types in order to identify the primary tumor site. The Pathwork Tissue of Origin Test received Food and Drug Administration (FDA) approval in 2008 to be used in fresh frozen tumor samples, and in 2010 to be used in formalin-fixed paraffin-embedded (FFPE) tumor tissue.