Summary
Definition
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 to 4 cm long and extends from the anal verge to the pelvic floor.[1] 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] 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]
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.
History and exam
Other diagnostic factors
- anal discharge
- anal itching
- fecal incontinence
- anal fistula
- non-healing ulcer
- inguinal node mass
- features of distant metastasis
Risk factors
- HPV
- HIV
- immunosuppression
- receptive anal intercourse
- multiple sexual partners
- autoimmune disease
- smoking
- anal intraepithelial neoplasia
- anal fistula
Diagnostic tests
1st tests to order
- anoscopy
- biopsy of tumor
- pelvis MRI scan
- pelvis CT scan
- abdominal and chest CT scan
Tests to consider
- PET scan
- inguinal node needle biopsy
- serum HIV enzyme-linked immunosorbent assay (ELISA)
- cervical cancer screening
Treatment algorithm
Contributors
Authors
Bruce D. Minsky, MD
Professor and Frank T. McGraw Endowed Chair
Radiation Oncology
MD Anderson Cancer Center
Houston
TX
Disclosures
BDM declares that he has no competing interests.
Peer reviewers
Brian Czito, MD
Associate Professor
Department of Radiation Oncology
Duke University Medical Center
Durham
NC
Disclosures
BC declares that he has no competing interests.
Lisa Kachnic, MD
Chief and Associate Professor
Department of Radiation Oncology
Boston University Medical Center
Boston
MA
Disclosures
LK has received funding from the US Department of Defense to perform research on non-anal cancers. LK is the author of a study referenced in this topic.
Sue Clark, MD
Consultant Colorectal Surgeon
Assistant Director
The Polyposis Registry
St Mark's Hospital
Harrow
UK
Disclosures
SC declares that she has no competing interests.
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