Cancer stem cells: targets and potential biomarkers for radiotherapy.

Cancer stem cells (CSC) have the unique ability to cause tumor recurrences if they survive treatment. Radiotherapy has curative potential because it has been functionally shown to sufficiently inactivate CSCs. It is well known that CSCs mediate the radiation resistance of tumors by tumor-specific factors, such as the pretreatment number of CSCs and repopulation or reoxygenation during fractionated radiotherapy. CSCs appear to have a higher intrinsic radioresistance than non-CSCs, a factor that is especially important for the development of predictive biomarkers that, if this finding holds true, can only be successfully established if they are stem-cell specific. Recent clinical data imply that stem-cell-related surface markers may be directly used as predictors for the radiocurability of tumors with comparable risk factors, such as histology and size. Future studies need to address the question of which additional markers need to be considered if more heterogeneous patient collectives are investigated. With the goal of developing a direct targeting approach, investigators are currently evaluating several drugs that are intended to target CSCs by inhibiting stem-cell-related signal transduction pathways. We need to preclinically test such drugs as combined-modality therapies in combination with radiotherapy to evaluate their curative potential, and optimize them by increasing their specificity to CSCs over normal tissue stem cells to avoid increased radiation toxicity.