Tumor angiogenesis is the process by which new blood vessels are formed from the existing vessels in a tumor to promote tumor growth (1). Although angiogenesis is essential for tumor growth and metastasis, tumorigenesis and malignant transformation of the tumor are not dependent on angiogenesis. Angiogenesis is a complex process that is mediated by several angiogenic and antiangiogenic factors produced by the tumor cells, the blood, and the stroma of the host tissue (1-3). The balance in the production of these factors can help predict when angiogenesis will develop; angiogenesis develops when the proangiogenic factors overcome the antiangiogenic factors. The process of angiogenesis includes endothelial proliferation, breakdown of basement membranes of the capillaries, endothelial cell migration into the extravascular space in the stroma, formation of capillary tubes, communication with the postcapillary venules, and blood flow through the new blood vessels (4-7). Tumor angiogenesis is characterized morphologically by an increase in the number of blood vessels, including new capillaries, capillary sprouts, nonendothelialized capillaries, and arteriovenous shunts (5-9). Some of these vessels mal-