What are the most important factors that should be considered when treating cancer? Many would argue that the process of metastasis is one, given that metastases are the cause of death for most cancer patients. The metastatic process is complex and it appears that the closer we get to understanding one mechanism or concept the more factors that should also be considered we uncover. Indeed, Klaus Pantel and colleagues (on p329) discuss the biological relevance of disseminating tumour cells to tumour metastasis, and suggest ways in which the detection and targeting of these cells might improve therapeutic intervention. But how do tumour cells disseminate from the primary tumour in the first place? John Pawelek and Ashok Chakraborty (p377) discuss the hypothesis that tumour cells might fuse with migratory cells in the surrounding stroma, enabling tumour cells to be motile and more likely to metastasize.

Others might suggest that the cancer stem cell theory is crucial, as this subpopulation of self-renewing tumour cells is thought to be sufficient to drive tumour growth and development, and these cells are more resistant to cancer therapy. Whether cancer stem cells are linked with the frequency of relapse in patients with chronic myeloid leukaemia after treatment with imatinib is discussed by Michael Savona and Moshe Talpaz on p341. That the Wnt signalling pathway is potentially involved in the regulation of stem cell self-renewal has sparked renewed interest in the link between developmental pathways and cancer, and Wnt is the subject of a Timeline article by Walter Birchmeier and Alexandra Klaus on p387.

Although the answer to the above question does depend on your viewpoint, our interest in all the questions surrounding cancer should continue to improve our understanding of how we should target this disease.