Molecular diagnosis of the tissue of origin in cancer of unknown primary site: useful in patient management.

OPINION STATEMENT: Cancer of unknown primary site (CUP) is a clinicopathologic syndrome consisting of many types of cancer and accounting for approximately 3 % of all patients with advanced cancers. This syndrome has frustrated patients and physicians for decades, because a primary site or tissue of origin has not been possible to identify clinically, despite the presence of metastatic tumor. Favorable subsets (approximately 20 % of all CUP) with a presumptive occult primary site have been recognized for several decades based on clinical and standard pathologic features; site-specific therapy in these patients improves their survival compared with the majority of other (approximately 80 %) CUP patients. These other patients, most with adenocarcinomas, have been difficult to treat because the tissue of origin was unknown. Broad-spectrum empiric chemotherapy became the standard approach for these patients in the past 30 years. More recently, new diagnostic technology (evolving immunohistochemistry and emergent gene-expression profiling) has enabled us to establish accurately a tissue of origin in most (90 %+) CUP patients. Gene-expression profiling assays complement standard pathology and for the majority of biopsy specimens accurately identify the primary site or tissue of origin; clinical studies have supported the value of site-directed therapy. When the tissue of origin is in doubt after standard pathologic examination, a gene expression assay is frequently diagnostic, and the outcome of many CUP patients is improved with site-specific therapy. The era of empiric therapy has ended in favor of site-specific therapy, based on the precise diagnosis of the tumor type present in each patient.