Bone metastasis (also referred as bone mets) basically occurs when the cancer cells break off from a primary tumor that is anywhere else in the body and then travel through the blood stream, or lymph system, to a part of the bone and become metastatic cancer cells. This is dissimilar to a primary bone tumor, which normally starts in the bone. When these cancer cells deposit themselves into the bone, they release substances that turn on either osteoclasts or osteoblasts. Osteoclasts will lead to the breakdown of the bone, without a new bone being made, which weakens the bones.