An uncommon cancer, anal cancer appears in the anal canal, the short tube at the end of the rectum through which stool passes out of the body. There are several types of anal cancer categorized by the types of tumors that develop. Squamous cell carcinomas are in the anus caused by squamous cells that line most of the anal canal; basal cell carcinomas are skin cancer that affects areas of the skin exposed to the sun; and gastrointestinal stromal tumors are likely to form in the stomach or small intestines.