Fine needle aspiration and core needle biopsy of metastatic malignancy of unknown primary site

Metastatic malignancies of unknown primary site (MUP) is the eighth most common form of malignancy, with an estimated 10–15% of oncology patients having a MUP. Fine needle aspiration cytology (FNA) and core needle biopsy (CNB) are often the first procedures utilized in the work-up of these cases and have a pivotal role for the diagnosis of metastases. There is an increasing emphasis on the precise classification of malignancy and determination of primary site of origin, utilizing smaller specimens. Recent available data suggest that there is a management benefit in identifying the primary site and/or specific cell lineage of MUP. In addition, the pathologists are asked to preserve the limited diagnostic material for potential molecular testing, as selected patients may benefit from targeted therapy. However, these tasks can become extremely challenging, especially if there is no previous history of malignancy, prior pathology is not available for review, or there is an unpredictable pattern of metastasis. In this review, we present a contemporary clinicopathologic approach to the work-up of MUP that includes cytomorphology, ancillary studies, and clinicopathologic correlation. The cytohistologic subclassification of malignancies into specific cell lineages and/or morphologic categories is presented. Knowledge of the various patterns of metastasis to common and unusual sites can help narrow down the location of a primary site. The use of ancillary studies with particular emphasis on IHC utilizing an algorithmic approach and the role of molecular analysis as a diagnostic and theranotic test are also discussed. When the cell block and/or CNB lacks sufficient material for ancillary testing, the cell transfer technique may be utilized.