The mysteries of cancer spread called metastases remain locked up inside the body. We only know the basics. Cancer can spread by direct extension. This means it just moves from one cell to the next.
Or it can spread from one body part to another. For example, there's nothing separating the abdominal cavity from the rest of the body. Cancer cells are free to move about.
The blood system and the lymphatic system are two other ways cancer can hitchhike throughout the body. Each cancer has its own special way of getting around. Some are more prone to get spread through the blood stream while others are more likely to spread via the lymphatics.
All the blood through the stomach and intestines also goes to the liver for filtering. For this reason colon cancer is often linked with liver cancer. The liver and the colon both rest up against the back of the abdominal wall. Only a sheath of muscle tissue separates them from the vertebrae.
Small blood vessels supplying the vertebra with oxygen and nutrients can also deliver tiny cancer cells to the bones of the spine. Knowing the typical modes of cancer metastases helps doctors trace the cancer to its origin.