Cancer stem cells--an old idea that's new again: implications for the diagnosis and treatment of breast cancer.

The medical treatment of solid tumors is beset by two fundamental problems: the fact that even striking initial responses are often followed by drug-resistant recurrences, and the lack of predictive tools to design individualized treatment strategies. These therapeutic problems have a biological basis in the genetic heterogeneity and genomic instability of solid tumors. Traditionally, these were thought to result from accumulated mutations in random tissue cells, leading first to transformation and eventually to loss of differentiation and the selection of drug-resistant clones. The cancer stem cell theory posits that tumors arise specifically from the transformation of rare tissue stem cells or progenitor cells, which generate the bulk of the cancer through proliferation and abortive differentiation akin to aberrant tissue self-renewal. Cancer stem cells are slow-dividing and inherently drug-resistant, and their eradication would be necessary for long-term success in cancer treatment. The authors present a brief overview of this theory, its potential implications and the evidence supporting it, focusing specifically on breast cancer.