Overview

The actual growth of a solid tumor depends on the development of its blood supply. Tumors 1-2 mm in diameter can receive nutrients by diffusion, and further expansion requires the development of new blood vessel. This phenomenon is known as Angiogenesis. Angiogenesis occurs in response to growth factors produced by growing tumors. In neoplasm, angiogenic factors get up-regulated while angiogenesis inhibiting factor get down regulated. Similarly, another phenomenon is lymphangio-genesis, in which the number of lymphatic vessel increases. Both Angiogenesis and lymphangio-genesis help in metastasis of tumor. Doubling time is the mean (average) interval between successive mitosis. It is characteristic of the particular type of tumor cell, like it is 2 weeks for acute leukaemia, 3 months in breast cancer, 24 hours for Burkitt's tumor etc. After 30-33 doubling time that is when the number of cells approximately equals to 109 to 1010, a tumor becomes detectable. The neoplasm becomes lethal when the population reaches approximately 5×1011 to 5×1012 cells that are after 39 to 42 doubling times (Barrow 2002). With most solid tumors, for example tumor of lung, stomach, uterus etc. rather than leukaemia (the tumor of white blood cells) the growth rate falls as the neoplasm get larger. This is partly because the tumor tends to outgrow its ability to maintain its blood supply, with resultant necrosis of some of its bulk, and partly because not all the cells proliferate continuously. Thus, cells of a solid tumor can be considered to belong to three compartments:

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