Spread is by way of the blood stream or by way of the lymphatics. Just as the arteries take blood from the heart to all areas of the body and the veins return it to the heart, there is also a network of lymphatic vessels that drain all areas of the body. The lymphatic vessels converge into regional sites containing nests of lymph nodes. The lymph nodes, or glands, filter the lymph fluid for infectious agents and mount an immune response against it. They are part of the immune system.
Cancer can spread by way of the lymph vessels and clog up the lymph nodes and start to grow.
Often the success or failure of being able to cure a cancer depends on whether the lymph nodes have cancer in them. Cancer is thought to arise in a single cell. The location of that cell is the site of the primary cancer. Cancers are named for the primary site of origin, i.e., skin, colon, breast, brain, etc. If a cancer is found in the liver that originated in the colon it is called metastatic colon cancer to the liver not a liver cancer.