Anal intraepithelial neoplasia (AIN)

Anal intraepithelial neoplasia (AIN) means there are abnormal cells in the lining of your anus. It is also called anal squamous intraepithelial lesions (SILs). This is because most of the abnormal cells are a type of cell called squamous cells.

A pathologist looks at your cells under a microscope to check whether your anal cells look different to normal anal cells. 

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

Diagram showing the anatomy of the anus

Grading of anal intraepithelial neoplasia (AIN)

AIN is divided into grades 1 to 3. The grade relates to how abnormal these cells look under a microscope.

  • AIN 1 – the cells are slightly abnormal.
  • AIN 2 - the cells are moderately abnormal.
  • AIN 3 - the cells are severely abnormal.

Another system doctors use refers to AIN or SIL as low grade or high grade:

  • In low grade SIL (LSIL or AIN 1) the cells are slightly different to normal anal cells.
  • In high grade SIL (HSIL or AIN 2 to 3) the cells are moderately to severely different from normal anal cells.

Treatment for AIN

Low grade SIL (AIN 1)

You might not need treatment. For some people treatment can relieve symptoms such as burning, itching, and bleeding. Your doctor will check your SIL by taking a sample of skin (biopsy). And they will arrange regular checks to monitor your SIL.

High grade SIL (AIN 2 to 3)

You will need treatment because the abnormal cells are less likely to get better on their own. They might develop into anal cancer, but the risk of them changing to a cancer is very small. Your doctor will discuss the best treatment option for you. Treatments might include:

  • laser to destroy the cells (laser ablation)
  • surgery
  • skin creams such as imiquimod or 5FU

Stages of anal cancer

  • AJCC Cancer Staging Manual (8th edition)
    American Joint Committee on Cancer
    Springer, 2017

  • Cancer: Principles and Practice of Oncology (11th edition)

    VT DeVita, TS Lawrence, SA Rosenberg

    Wolters Kluwer, 2019

  • Premalignant and Malignant Perianal Lesions

    M Abbass and M Valente

    Clinics in Colon and Rectal Surgery, September 2019. Volume 32, Issue 5, Pages 386-393

  • Anal cancer: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up

    S Rao and others

    Annals of Oncology, 2021. Volume 32, Issue 9, Pages 1087-1100

  • Anal squamous intraepithelial lesions: Epidemiology, clinical presentation, diagnosis, screening, prevention, and treatment

    UpToDate website

    Accessed July 2022

Last reviewed: 
01 Dec 2022
Next review due: 
01 Dec 2025

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