Scientists have learned more about the way cancer spreads from one place in the body to another. Once it spreads, it gets more difficult to treat. Cancer is a disease of uncontrolled cell growth. Traditional thinking is that cells simply break off the main cancer, travel through the blood and grow in other organs. But the new findings suggest there is more to this process. Researchers found that cells from the main tumor send out messengers. These prepare the new organ for a secondary tumor (瘤). It works this way. The primary tumor releases proteins called growth factors into the blood. They signal cells at the target organ to produce a sticky protein, called fibronectin. It attaches to the suce of bone marrow (骨髓) cells. The result is a kind of landing area for cancer to arrive and grow into a secondary tumor. The bone marrow cells help make vessels for blood to pass through and feed the cancer cells. The researchers believe that without the bone marrow cells, the tumors could not land on the new organ and grow. Scientists from Cornell University in Ithaca (伊萨卡岛), New York, led the research. The study appeared in the publication Nature. It involved laboratory mice. The researchers killed off bone marrow cells in the animals and replaced them with special new ones. The bone marrow cells were brightly colored, so the researchers could observe them. The mice were then injected with lung cancer cells. The researchers were surprised when the bone marrow cells reached the lungs days before any cancer cells. They found that the bone marrow cells arrived to prepare the lungs for the cancer to spread. Other tests led to similar findings. The researchers believe they would find the same results in humans. They say knowledge of the process could lead to new ways to fight cancer. Cancers can be caused by genetic or environmental conditions, or a combination. There are estimates that about one-third of cancer deaths could be pred if people took better care of themselves. That includes better diet, exercise and no smoking. Researchers recently studied the causes of the seven million cancer deaths worldwide in two thousand one. They linked nine avoidable risks to almost two and one-half million of them. The study led by Harvard researchers in the United States appeared in the Lancet. According to scientists, what will happen to the tumor if there is no bone marrow