Tumors of Unknown Primary

Abstract

Not so long ago, “Anaplastic Tumor” ranked among the top ten most common diagnoses in pathology departments around the World. That this is no longer true is in large measure attributable to the widespread use of IHC methods for the diagnosis of very poorly differentiated or anaplastic malignant tumors, whether as metastases or apparent primary lesions. In effect, IHC stains provide a sensitive method for the demonstration of characteristic cell products or structural proteins (antigens) that allow identification of cell lineage when morphologic features alone cannot do so. This chapter provides a concise and practical approach for the application of IHC methods to facilitate the diagnosis of tumors of unknown origin. Screening approaches employing selected panels of stains are described for common broad categories of tumors (carcinomas, sarcomas, lymphomas, etc.), together with “second-tier” stains to home in on a more specific diagnosis. Emphasis is on an efficient practical approach, that is both economical (using the minimum number of stains) and rapid (requiring of the least number of additional staining runs). For rapid reference many of the IHC stains and stain panels are presented in summary tables, without further detail of the typical patterns of reactivity of each antibody. However, all of the antibodies (stains) described in this chapter have been discussed previously in; further details may be found by recourse to individual organ and issue specific chapters throughout the book.