What are malignant tumors?

Malignant tumors grow by invading nearby cells and spread to other parts of the body through a process called metastasis. Cells break off the tumor, enter the bloodstream or lymphatic system, and spread to another area, infecting additional tissue. This is how a tumor which starts in one part of the body, such as the breast or prostate, can spread to another type of tissue, such as the bones.