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AIN is short for anal intraepithelial neoplasia and it is often referred to as ‘anal precancer’, as some forms of it can be serious, but, it is important to be clear that it is not cancer

  • Neoplasia: microscopic cell changes seen as cells undergo the process of becoming cancerous
  • Intraepithelial: the abnormal cells have not got further than the epithelium or the lining of the anus

If you have been told you have AIN it is likely that it has been found due to a biopsy that has been taken, on an anal smear test, or, on looking with a microscope at the peri-anus (the skin around the anus) and the anal canal.

What causes AIN?
In 90% of cases, AIN is linked to infection with the human papillomavirus (HPV). HPV infection is extremely common. Not everyone who comes into contact with the HPV virus will get AIN. HPV virus also causes warts. Not everyone who has had warts will develop AIN.

Why do some people infected with HPV get AIN and some do not?
Not everything is known yet about AIN.
We know that immunosuppression of any kind weakens the body’s defences against the HPV virus and makes AIN more likely, often some years later. This includes

  • HIV infection (even if well-controlled)
  • Organ transplants
  • Inherited immune defects
  • Immune-suppressing drugs 

AIN is more common in those who practise receptive anal sex, especially men who have sex with men (MSM), and are HIV positive.
Smoking is a known risk factor for AIN and anal cancer.
However, some people have AIN without any risk factors.

It is hoped that the HPV vaccine will eventually make AIN rare, however as far as we know at present, once you have been infected with HPV (which most sexually active people have been), the vaccine will not work to treat or prevent AIN.

How should AIN be treated?
A study (LOPAC) is underway to compare treatment with close surveillance of high grade AIN in those immunosuppressed patients at highest risk, to answer the question as to which is the best way to prevent high grade (HSIL) becoming cancer.

At present, we suggest

  1. If you have low-grade AIN then you do not need treatment. We will suggest that testing is repeated in the future.
  2. If you have high grade AIN or HSIL of the anus then we will suggest follow up with HRA (high resolution anoscopy).
  3. For persistent areas, and wide areas of AIN, topical treatment (creams you apply yourself) can be used: imiquimod (Aldara) and 5 fluorouracil are both types of cream that can be prescribed for use 2-3 times weekly for 3-4 months. This treatment is most successful for AIN in the skin around the anus.
  4. In some cases, laser ablation of the AIN/HSIL is suggested. We are waiting for the results of the LOPAC study to give us the scientific evidence that will show whether laser intervention is more likely to prevent anal cancer than just close observation.

More information


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