The word epithelium is synonymous with the words lining or covering. This major group of cancers arise from structures that are in contact with the environment. You can trace a continuous path from the squamous epithelium of the face to the squamous epithelium of the mouth, down the squamous epithelium of the throat and esophagus onto the glandular epithelium of the stomach, then through the small intestine which is also lined with a glandular type of epithelium to the large bowel, also glandular epithelium, and out the anus which is squamous epithelium. All of this is in contact with the external environment. A similar path can be traced from the breast through the nipple into a milk duct and finally into a milk gland.
Cancer can spread to other parts of the body.
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.