When cancer occurs in the body, the place where it first starts is known as a primary tumour. A malignant (cancerous) tumour is made up of millions of cancer cells. Some of these cells may break away from the primary cancer and be carried in the bloodstream to another part of the body, in this case the bones. The cancer cells settle in the bone and may form a new tumour, which is referred to as a secondary cancer or a metastasis.

Secondary bone cancer does not start in the bone, but is the result of cancer cells spreading to the bone from a primary tumour. Sometimes only one area of bone is affected, but in other people a number of bone secondaries develop, often in different bones in the body. Not all the secondaries will cause symptoms or problems.

Although any type of cancer can spread to the bone, the most common types are cancers of the breast, prostate, lung, thyroid and kidney. People who develop secondary bone cancer usually know that they have a primary cancer, although occasionally a secondary bone cancer is found before a primary cancer is diagnosed. If the primary cancer cannot be found it is called an unknown primary tumour.

Primary bone cancer, where cancer begins in the bone itself, is a completely different type of cancer with very different treatments.