Cancer begins when cells in the body start to develop wild. Cells in almost any piece of the body can progress toward becoming cancer, and can spread to different parts of the body. To take in more about how cancers begin and spread, see What Is Cancer Anal cancer begins in the rear-end. To comprehend Anal cancer, it thinks about the butt and how it works.Anal cancer is a cancer (threatening tumor) which emerges from the rear-end, the distal opening of the gastrointestinal tract. It is a particular substance from the more typical colorectal cancer.