Cancer stem cells (CSC) are a sub-population of cancer cells that possess characteristics associated with normal stem cells (SC), such as self-renewal and the ability to differentiate into multiple cell types. CSC are tumorigenic (tumor forming), in contrast to the bulk of cancer cells, which are thought to be non-tumorigenic. CSC persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumors. The study of CSC has several implications for future cancer treatment and therapies. The CSC hypothesis infers that if the CSC were eliminated, the tumor would simply regress due to differentiation and cell death. By selectively targeting CSC, it would be possible to treat patients with aggressive, non-resectable tumors, as well as to prevent the tumor from metastasizing. Although the term CSC may be appealing, a more functional definition that includes terms such as “tumor-initiating cell” or “cancer-initiating cell” as proposed by some groups, may be more appropriate. The implication therefore is that CSCs are derived from normal stem cells that acquire the genetic hits necessary for malignant transformation. Normal SC appear to be the ideal candidates of cancer transformation because cells with long life spans are more likely to accumulate the appropriate number and variety of genetic lesions necessary to acquire full tumorigenic capacity. In addition, normal SC, which are already indefinitely self-renewing, need a fewer number of mutations to be transformed in malignant cells.

It has been repeatedly shown by us and others that treatment with standard chemotherapeutic agents can lead to an increase in the numbers of stem cells, with the implication that the risk of subsequent recurrence or metastasis is increased. Therefore combination treatments will be necessary to truly change the way in which cancer is managed - drugs to reduce the size of the tumour mass, combined with drugs to prevent the stem cells regenerating it in future.