Carcinoma is a type of cancer that starts in the epithelium. The epithelium is the lining of the body. The skin is part of the epithelium, and so is the inside of blood vessels. The intestines are lined with epithelium; in fact, any hollow organ or space in the body is lined with epithelial tissue and is part of the epithelium. For this reason, carcinomas can occur in many places in and on the body.