This section is from the "Health and Survival in the 21st Century" book, by Ross Horne.
Cells do not have to be fully de-differentiated to grow as cancer, and therefore pathology tests can usually identify them with the tissue of their origin. Thus, when cancer cells migrate in the blood and lymph and start secondary tumors elsewhere in the body (metastatis), the site of the primary growth can usually be determined by examination of cells from the secondary. The secondary growths are the most fast growing, because although at the tissue of their origin the normal constraints to growth still tend to control the primary cancer, away from the tissue of origin the constraints do not exist. Primary tumors therefore are usually slow growing, and it is only when metastasis occurs that cancer is considered to be terminal.
As the cancer growth proceeds, the process becomes a vicious circle in which more and more of the body's supply of blood sugar is squandered in the wasteful production of lactic acid and so the entire body, poisoned and starved of sustenance, wastes away in the condition known as cachexia.