There are a billion miles of blood vessels the human body. Just by the law of chance, if cancer traveled through the blood, some spots would develop in other places and not all in the bones. Breast cancer goes to the bones first, because the mold of breast cancer feeds on calcium. Therefore, the spread of cancer is not random. The mold can only grow in certain pHs and chemistry of the cells.
Cancer can only grow when the cells are acid, the blood is alkaline, and there is poor oxygenation.
Every day old cells die and new cells are being created. The natural death of the old cell is called apoptosis. As the old cell becomes weakened, its DNA and RNA start to break up. The body contains proteins, called cytokines. These are analogous to hormones of the immune system.