The sponge-like substance inside your bones is called marrow. Cells, which can grow into red blood cells (RBCs), white blood cells (WBCs), or platelets, are found deep inside the marrow. The improper growth of cells in the marrow is known as bone marrow cancer . It is often known as blood cancer and it is not the same as bone cancer. Cancers that spread to bones and bone marrow aren't considered bone cancers. Cancer that begins in the bone marrow is called Primary Bone Cancer . Cancer that spreads to the bone from another part of the body is called Secondary Bone Cancer . Osteosarcoma is the most common type of primary bone cancer . Cancerous cells develop in the bone marrow and crowd out the healthy blood cells. This can cause a decrease in the number of red blood cells, white blood cells, and platelets in your blood. A decrease in these blood cells can lead to fatigue, increased infections, and easy bleeding. Treatment for bone marrow cancer often involves surgery, chemot...