Lesions of uncertain differentiation

Abstract

As one may expect, the lesions of unknown differentiation constitute a very heterogeneous group of both neoplasms and tumor-like 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. For a lot of tumors, discussed in this chapter, the line of differentiation that they are recapitulating is not clear. In contrast, for some other tumors, although the line of differentiation can be identified, the cellular counterpart cannot be identified in normal mesenchymal tissues. Consideration of their local growth pattern and clinical behavior allows further distinction between benign and malignant lesions, as presented below.