Algorithmic approach of immunohistochemistry in metastatic carcinoma of unknown primary site

Cancers of unknown primary site (CUPs) are histologically confirmed metastatic carcinoma for which primary site cannot be identified after standard diagnostic approach. It comprises 3-4% of all malignant neoplasms. The broad tumour type for CUPs is carcinoma. Metastatic tumours are more difficult to classify than primaries by Immunohistochemistry (IHC). Integration of morphology & IHC is the gold standard at diagnosing CUP. It is important to be aware of possible pitfalls of IHC and pay attention to correct interpretation. One should have a working algorithmic approach to identifying and classifying CUPs.