Anal cancer

Anal cancer is associated with human papillomavirus infection and common in men who have sex with men and in immunosuppressed patients, especially people living with HIV.

The most common type of anal cancer is squamous cell carcinoma.

Presenting symptoms are typically bleeding and anal pain.

The cure rate is high (75% to 90%).

Standard treatment for anal cancer is chemotherapy plus radiation therapy. Surgery or immunotherapy may be an option for some patients.

More intensive chemotherapy and radiation has not improved results.

Cancers of the anus can occur in three regions: the perianal skin, the anal canal, and the lower rectum. In most patients, the anal canal is 3-4 cm long and extends from the anal verge to the pelvic floor.[1]Simpson JA, Scholefield JH. Diagnosis and management of anal intraepithelial neoplasia and anal cancer. BMJ. 2011 Nov 4;343:d6818. http://www.ncbi.nlm.nih.gov/pubmed/22058137?tool=bestpractice.com The anal verge is the junction of the anal canal, where the mucosa does not contain hairs or glands, with the hair-bearing perianal skin. The perianal skin extending 5 cm distal from the anal verge is called the anal margin.[1]Simpson JA, Scholefield JH. Diagnosis and management of anal intraepithelial neoplasia and anal cancer. BMJ. 2011 Nov 4;343:d6818. http://www.ncbi.nlm.nih.gov/pubmed/22058137?tool=bestpractice.com A clear anatomic distinction between the anal canal and the anal margin is needed because of the different natural histories of cancers that arise in these two areas. There is considerable confusion when comparing series in the literature because of the different definitions used for the anal canal and the anal margin.

To clarify this issue, the American Joint Committee on Cancer and the Union for International Cancer Control agreed that the anal canal extends from the anorectal ring to the anal verge. This is an important distinction because these two governing bodies also agreed that anal margin tumors behave in a similar fashion to skin cancers and should be classified as skin tumors and treated as such. However, if there is any involvement of the anal verge by tumor, it should be treated as an anal canal cancer.

[Figure caption and citation for the preceding image starts]: Cross-sectional anatomy of the anal canal. 1 indicates an area of transitional mucosa; 2 indicates an area of squamous epithelium devoid of hair and glands. The perianal skin extending 5 cm distal from the anal verge is called the anal margin.Created by BMJ Evidence Centre [Citation ends].

The most common anal malignancy is squamous cell carcinoma.[1]Simpson JA, Scholefield JH. Diagnosis and management of anal intraepithelial neoplasia and anal cancer. BMJ. 2011 Nov 4;343:d6818. http://www.ncbi.nlm.nih.gov/pubmed/22058137?tool=bestpractice.com [2]Morris VK, Kennedy EB, Amin MA, et al. Systemic therapy for stage I-III anal squamous cell carcinoma: ASCO guideline. J Clin Oncol. 2025 Feb 10;43(5):605-15. https://ascopubs.org/doi/10.1200/JCO-24-02120 http://www.ncbi.nlm.nih.gov/pubmed/39680825?tool=bestpractice.com

Adenocarcinomas can arise from anal crypts and should be treated as a rectal cancer. Because of their location and their higher lymphatic flow compared with rectal adenocarcinomas, anal crypt adenocarcinomas have a higher risk of inguinal node spread. Squamous cell and cloacogenic histologies are collectively defined as anal canal cancers.

Other rare cancers can develop in the anal canal, including small cell neuroendocrine carcinomas, lymphomas, and melanomas.