Metastatic Tumors of Unknown Origin: Ancillary Testing in Cytologic Specimens

The application of ancillary studies, such as immunostains, to cytopathology has improved the ability to make accurate diagnoses with precise subclassification. Even with these techniques, there are still aspiration and exfoliative cytology cases for which it remains difficult to definitively determine the source and/or subtype. This article focuses on the well-established and novel ancillary studies used in the modern era of cancer diagnoses in cytopathology, particularly in the diagnostic work-up of metastatic tumors without a known primary.