Metastasis is the process by which cancer spreads from its primary site of origin to other places in the body. Tumor cells escape from the primary tumor and travel in the blood stream or lymphatics to reach distal sites such as the skeleton.
Skeletal metastases from carcinomas are the most common malignant tumors involving bone, far more common than primary bone tumors. Solid organ cancers most likely to spread to bone include cancers of the breast, lung, thyroid, kidney, and prostate. Blood cell cancers such as lymphoma and multiple myeloma are also commonly detected in the skeleton.
Metastatic lesions are of course not made of normal bone tissue and therefore are at risk for a so-called pathological fracture. (The term “pathological fracture,” seemingly redundant, refers to fracture in a bone that is itself not normal, as discussed below.)
Typically, skeletal metastases are osteolytic (Figure 1); the tumor cells in the bone increase osteoclast activity, eroding the bone. This bone lysis can cause pain, increase the risk of fracture, and produce hypercalcemia as calcium is released from the breakdown of mineralized bone.
Lesions can also be osteoblastic, that is, characterized by increased bone formation. Osteoblastic metastases are common in prostate cancer. Metastatic lesions can also be mixed (i.e., osteoblastic and osteolytic), as may be seen with breast cancer. Though the risk of fracture is greatest with an osteolytic lesion, the bone architecture in osteoblastic or mixed lesions is also abnormal and thus prone to fracture as well.
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