Cancer spread. Cancer cells have broken away from the primary cancer (where the cancer began in the body) and have spread to another organ or part of the body, where they have begun to grow. Secondary cancer has to be treated according to the type of cells that it is made up of. For example, breast cancer cells that have spread to the lung will respond to breast cancer treatments and not lung cancer treatments because the cancer cells are breast cancer cells no matter where in the body they are growing.