The anus is the last few centimeters of the gastrointestinal tract which connects the rectum to the skin surrounding the anus, called the perianal skin. Stool leaves the body through the anus. These abnormal cells continue to divide to make more cells, even though the body does not need the new cells. After a while, the accumulation of unneeded new cells results in a mass of extra tissue called a growth or tumor.
Anal tumors can develop in the anus.
Cancerous or malignant tumors often outgrow their original site and spread to nearby tissues or organs by entering the bloodstream or lymphatic system. The spread of cancer cells to other areas is called metastasis. Anal cancer is uncommon, affecting about 3,000 Americans each year. The cause of anal cancer is only partially understood. It is known that people who have had anal or genital warts, which is transmitted by a virus called the human papillomavirus, are at increased risk of developing anal cancer.