Cancer cells can spread locally by moving into nearby normal tissue. Cancer can also spread regionally, to nearby lymph nodes, tissues, or organs. And it can spread to distant parts of the body. Single cells from a tumour can break away from a tumour and migrate away from the original site via the blood stream.
Once in the blood, they can go to any part of the body. Many of these cells die, but some may settle in a new area, begin to grow, and form new tumors. This spread of cancer to a new part of the body is called metastasis.
Why does cancer come back? Well the most likely reason for cancer reoccurrence is that some cancerous cells escaped destruction during treatment, and survived. Unfortunately, if even a single cancer cells survives the treatment it can in time divide over and over to form a tumour once again.