The anus is the opening at the end of your rectum. Anal cancer can start anywhere in the anus. Where it starts The cause of anal cancer is unclear. Having many sexual partners and having anal sex are both major risks. The rectum is the bottom section of your colon (large intestine). Age (55 and older); Anal sex; Sexually transmitted diseases; Multiple sex partners The following symptoms can be seen with anal cancer, but may also be caused by less. Anal cancer can cause signs and symptoms such as rectal bleeding People who engage in anal sex have an increased risk of anal cancer.