Metastatic tumors are cancers that started in another location and spread to the bones. More than 90% of all these metastatic lesions in bone are caused by a small number of primary tumors including breast, lung, prostate, thyroid, kidney and multiple myeloma. Bone metastases occur in up to 70 percent of patients with advanced breast or prostate cancer and in approximately 15 to 30 percent of patients with carcinoma of the lung.

Metastases can be characterized as osteolytic or osteoblastic, representing two extremes in which dysregulation of the normal bone remodeling process occurs. Patients can have both types or mixed lesions. Patients with breast cancer have predominately osteolytic lesions, although at least 15 to 20 percent of them have predominately osteoblastic lesions. Secondary formation of bone can occur in response to bone destruction, making it possible to detect osteolytic lesions by means of bone scanning, which identifies sites of active bone formation. Only in multiple myeloma are the lesions purely lytic. In prostate cancer patients, the lesions are predominately osteoblastic, although there is also increased bone resorption in these osteoblastic lesions, allowing agents that block bone resorption to reduce bone pain and risk of pathologic fractures in these patients.

Factors accounting for the frequency of bone metastases:

• High blood flow in areas of red marrow
• Adhesive molecules produced by tumor cells bind to marrow stromal cells and bone matrix
• The adhesive interactions cause the tumor cells to increase production of angiogenic and bone-resorbing factors that aid tumor growth
• Large repository for immobilized growth factors, released and activated during bone resorption