Cancers that arise from supporting structures such as bone, muscle, cartilage, fat and fibrous tissue are called sarcomas. The third major category are the leukemias and lymphomas. These cancers arise from blood cell elements. Brain cancers, nerve cancers, melanomas, and certain testicular and ovarian cancers do not fall into any general category. Carcinomas arise from the epithelial covering or epithelial lining of structures.
Skin is an epithelial lining of the exposed parts of the body.
The intestines are lined with a glandular type epithelial lining. 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.