Do we know what causes bone metastases?

How cancer cells spread

For cancer cells to spread to other parts of the body, they have to go through several changes. They have to be able to break away from the original tumor and enter the bloodstream or lymph system, which can carry them to another part of the body. At some point they need to attach to the wall of a blood or lymph vessel and move through it into a new organ. They then need to be able to grow and thrive in their new location. All the while, they need to be able to avoid attacks from the body's immune system. Going through all these steps means the cells that start new tumors may no longer be exactly the same as the ones in the tumor they started in.

Why some cancers spread to bones

Where a cancer metastasizes depends on its exact type and where it started in the body. Some cancer cells carry substances on their surfaces that help them stick to different organs. Cancer cells that tend to spread to bone may attach better to the cells and supporting network in bone. The cancer cells may release chemicals that affect how the bone cells work, making it easier for the cancer cells to get a foothold in the bone. The bone cells themselves may also release hormone-like factors that help some cancer cells grow.

Discoveries about the interactions between cancer cells and normal bone cells are being used to develop new ways to treat or even prevent bone metastasis.

What happens when cancer grows in bones?

Cancer cells can affect bones in 2 ways.

• Often, the cancer cells make substances that activate bone cells called osteoclasts. These bone cells in turn dissolve and weaken the bones. The holes that develop when parts of bones dissolve are called osteolytic or lytic lesions.
• Sometimes, the cancer cells release substances that promote bone formation by cells called osteoblasts. This makes areas of the bones harder, a condition called sclerosis. The areas in bones where this occurs are called osteoblastic or blastic lesions.

Both lytic and blastic types of bone metastases can cause pain. Bone metastasis is one of the most frequent causes of pain in people with cancer.

When the cancer dissolves bones, they become weak and can break (fracture) easily. Fractures occur much less often with cancers that cause sclerosis.

When cancer spreads to the bones of the spine, it can press on the spinal cord. This can cause nerve damage that may even progress to paralysis if not treated.

As cancer cells damage the bones, calcium from the bones is released into the blood. This can lead to problems caused by high blood calcium levels, called hypercalcemia.

Bone metastasis can also cause other problems that can make it hard to keep up your usual activities and lifestyle.