[Immunohistochemical diagnosis in cancer metastasis of unknown primary tumor]

Immunohistochemical studies on metastatic carcinomas of unknown primary site are cost-effective and often allow a specific identification of the tumour origin, especially if the metastases are adenocarcinomas by light microscopy. Commercially available site-specific markers include prostate-specific antigen, thyroglobulin, thyroid transcription factor-1, uroplakin III, GCDFP-15, oestrogen and progesterone receptors, alpha-fetoprotein, the A103 monoclonal antibody against MART-1, cytokeratins 7 and 20, cytokeratins of basal cell type, p63, carcinoembryonic antigen, CA125, EMA, vimentin, HepPar-1, WT-1 and S100 protein. However, immunostaining with most of these markers does not show an absolute specificity for a certain primary site. For this reason, histopathologists interpretating staining results with these markers should take the available clinical data and the histological features of the metastatic carcinoma into consideration. These data are necessary to estimate the relative a priori probability of possible carcinomas. Based on Bayes' theorem, the a priori probability can then be used to calculate the diagnostically relevant predictive values for immunostaining results with the chosen markers.