Tumors of uncertain differentiation

Abstract

As one may expect, the lesions of unknown differentiation constitute a very heterogeneous group of both neoplasms and tumorlike lesions. In the past, these tumors were also labeled as being “of uncertain origin.” In view of the knowledge that these tumors do not arise from their normal cellular counterparts, the former classification based on “histiogenetic” concepts was no longer valuable. The current classification is based on terms of “differentiation,” which depends on patterns of gene expression. In this book chapter, benign lesions such as intramuscular myxoma, intermediate tumors such as atypical fibroxanthoma, and malignant lesions such as synovial sarcoma will be discussed. This differentiation is predominantly based on their local growth pattern and clinical behavior, because of their often still unclear line of cell differentiation.