A-011 16 At initiation, tumours in a pre-vascular phase are supplied with oxygen and nutrients that diffuse from pre-existing normal vessels. When the tumour reaches a critical size of approximately 1-4 mm diameter, the resultant ischaemia leads to the secretion of angiogenic factors. These factors, such as vascular endothelial growth factor (VEGF), recruit and maintain tumour vessels. "New" vessels (neovasculature) exhibit increased blood volume and permeability compared with normal vessels.