Anal intraepithelial neoplasia (AIN)

Anal intraepithelial neoplasia (AIN) means there are abnormal cells in the lining of your anus. A pathologist looks at your cells under a microscope to check whether your anal cells look different to normal anal cells. 

It is not cancer but the cells might develop into cancer in the future. 

Most AIN cells start in the skin around the back passage (perianal skin), but they can also be found inside your anal canal.

Diagram showing the anatomy of the anus

Anal intraepithelial neoplasia (AIN)

There are different grades of AIN. The grade relates to the thickness of the surface layer of skin that is affected by abnormal cells.

  • AIN 1 means the abnormal cells are in the lower one third of the skin covering the anus
  • AIN 2 means the abnormal cells are in the lower two thirds of the skin
  • AIN 3 means the abnormal cells are in the full thickness of the skin of the anus

Your doctor might also refer to AIN as low grade or high grade:

  • In low grade AIN (AIN 1 and 2) the cells look slightly different to normal anal cells
  • In high grade AIN (AIN 3) the cells look very abnormal - AIN 3 is also sometimes called carcinoma in situ or stage 0

Treatment for AIN

Low grade AIN (AIN 1 and 2)

You might not need treatment. The abnormal cells can go back to normal on their own, without any treatment. Your doctor will check your AIN by taking a sample of skin (biopsy). And they will arrange regular checks to monitor your AIN.

High grade AIN (AIN 3)

You will need treatment because the abnormal cells are less likely to get better on their own. You usually have surgery to remove some of the skin around the anus.

Clinical trials have looked at other treatments including lasers to destroy the cells (laser ablation) and skin ointment (imiquimod).

Stages of anal cancer

Last reviewed: 
30 Apr 2019
  • AJCC Cancer Staging Manual (8th Edition)
    M Amin and S Edge.
    Springer, 2017.

  • Guidelines for management of anal intraepithelial neoplasia
    J Scholefield and others
    Colorectal Dis, 2011. Volume 13, Pages 3-10.

  • Diagnosis and management of anal intraepithelial neoplasia and anal cancer
    J Simpson and others
    British Medical Journal, 2011.

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