[Comparing immunohistochemical diagnosis of cancer of unknown primary with gene expression-based tumor classification].

Five percent of cancer cases present as metastases. If the primary tumor cannot be identified after diagnostic workup, the disease is referred to as cancer of unknown primary (CUP) and is classified as C80.9 according to ICD-10. In Germany, CUP is among the ten most common causes of tumor-related death, with mortality similar to mortality in gastric or pancreatic cancer. Biopsies of the tumor manifestation are generally examined histopathologically and by immunohistochemistry (IHC). Gene expression profiling (GEP) is a new diagnostic technique that might further contribute to tumor specification.In a retrospective study, 43 CUP cases underwent central immunohistochemical review and centrally performed GEP using a classifier based on 495 genes. There was concordance between IHC, GEP and clinical picture in 54% of cases. In four cases, the combination of methods led to an unequivocal identification of the primary tumor.In conclusion, we regard detailed IHC workup and complementary GEP advisable for the purposes of targeted therapy, as well as to identify or exclude specific tumors in a CUP situation.