Accessing the vasculature in cancer: revising an old hallmark.

The classic cancer hallmark, inducing angiogenesis, was born out of the long-held notion that tumours could grow only if new vessels were formed. The attempts, based on this premise, to therapeutically restrain angiogenesis in hopes of controlling tumour growth have been less effective than expected. This is partly because primary and metastatic tumours can grow without angiogenesis. The discovery of nonangiogenic cancers and the mechanisms they use to exploit normal vessels, called 'vessel co-option,' has opened a new field in cancer biology. Consequently, the cancer hallmark, 'inducing angiogenesis,' has been modified to 'inducing or accessing vasculature.'