Both primary tumors and metastases, for their growth, formation of new blood vessels i.e., angiogenesis is necessary. The new blood vessels supply the tumor with nutrients and oxygen. In absence of new blood vessels the tumor would roughly become a sphere of 2 mm diameter and a death in its center.
The mechanism of angiogenesis is not clearly understood, although degradation of basal lamina that surrounds a nearby capillary, migration of endothelial cells and formation of new basement membrane around them are the steps described for the formation of new capillaries in the tumors.
Many tumors have been found to secrete basic fibroblast growth factor (bFGF), transforming growth factor-? (TGF?) and vascular endothelial growth factor (VEGF) etc. All of these substances are angiogenic. Formation of new blood vessels directly enhances the growth of the tumor and provides an easier escape route for metastasis.
It is found that surgical removal of primary tumor stimulates the growth of metastatic tumor, possibly the primary tumors secrete a substance that inhibits angiogenesis of the secondary tumors. But it seems to be a mysterious aspect.