I figure that since cancer cells are just mutations of our regular cells, they have much the same antigen profile as the regular cells, enabling them to (usually) escape detection by our immune system.
Dr. Thomas Seyfried thinks macrophages merge with pre cacerous cells. Macrophages can go anywhere, so this is how cancer spreads.
His main theory for treating all forms of cancer is to deprive it of its two main sources of fuel, glucose and glutamine.
Very good, non technical explanation here:
https://www.youtube.com/watch?v=Yyt3Do4w7fs