The paradox of response and survival in cancer therapeutics.

Although most patients with cancer respond to therapy, few are cured. Moreover, objective clinical responses to treatment often do not even translate into substantial improvements in overall survival. For example, patients with indolent lymphoma who achieved a complete remission with conventional-dose therapies in the prerituximab era did not experience a survival advantage over similar patients treated with a "watch and wait" approach. Several studies have also shown that neither the magnitude nor the kinetics of clinical response has an impact on survival in multiple myeloma. Recent data suggesting many malignancies arise from a rare population of cells that exclusively maintains the ability to self-renew and sustains the tumor (ie, "cancer stem cells") may help explain this paradox that response and survival are not always linked. Therapies that successfully eliminate the differentiated cancer cells characterizing the tumor may be ineffective against rare, biologically distinct cancer stem cells. New methods for assessing treatment efficacy must also be developed, as traditional response criteria measure tumor bulk and may not reflect changes in rare cancer stem cell populations. In this article, we discuss the evidence for cancer stem cells in hematologic malignancies and possible ways to begin targeting these cells and measuring clinical effectiveness of such treatment approaches.