**1.2 Bone remodeling**

Bone tissue is normally constantly renewed and replaced, meaning that osteoclasts absorb old bone and osteoblasts build new bone. This process is called bone remodeling. Normally, these two processes are in balance with each other. Any abnormality in the function of osteoblast and osteoclast cells can cause this balance to be lost, resulting in a change in the resorption or formation of bone by these cells. Bone metastasis is one of the factors that upset this balance. Bone metastases are divided into three types: osteoblastic, osteolytic, or a combination of both [10–12].

### **1.3 Osteoblastic and osteolytic metastases**

The origin of various osteoblastic and osteolytic metastases is not yet well understood. Osteoblastic metastasis occurs when, under certain factors, the bone formation activity of osteoblast cells exceeds the bone-eating activity of osteoclast cells, and bone formation becomes greater than bone resorption. Osteolytic metastasis also occurs when the activity of osteoclasts exceeds that of osteoblasts and bone resorption becomes greater than bone formation [11, 12].
