New research from researchers at Mt. Sinai Medical Center may change the way we understand how cancer spreads (metastasizes). The study looked at how very early breast cancer spreads. Even before a tumor was visible, changes in immune cells called macrophages lead to movement of breast cancer cells to distant locations, including the lungs. These results differ from the common belief that cancer spreads only later in the disease. It could explain why some cancers are so hard to treat.