Anal cancer is relatively rare, making up only about 4 percent of lower gastrointestinal (GI) cancers. Several different types of anal cancer exist, but squamous cell carcinoma (cancer of the middle and outer skin cell layers) is the most common. Squamous cell carcinoma usually begins in the anal canal, from the anus and going inward about 2-3 centimeters.
Other less common types of anal cancer include:
- Adenocarcinoma. This cancer begins in cells of the lining of the anal canal or the glands just underneath it.
- Carcinoid tumor. This is a rare type of slow-growing anal cancer.
- Melanoma. This cancer begins in the melanocytes (cells that give skin its color).
- Small cell carcinoma. This is a rare type of aggressive anal cancer.
Causes of anal cancer
The exact cause of anal cancer remains unknown; however, researchers have identified several risk factors, including:
- Lowered immunity, including being HIV-positive
- Presence of condyloma or high-grade dysplasia
- Receiving anal intercourse
- Sexual activity, especially with multiple partners
- Sexually transmitted diseases (STDs), especially human papillomavirus (HPV)