Low grade squamous intraepithelial lesion of the anus

by Jason Wasserman MD PhD FRCPC
July 6, 2023


What is low grade squamous intraepithelial lesion of the anus?

Low grade squamous intraepithelial lesion (LSIL) is an abnormal growth that starts from cells that cover the inside of the anus and anal canal. Although the overall risk is low, LSIL is considered a precancerous condition because it can turn into a type of anal cancer called squamous cell carcinoma over time. Another name for LSIL is anal intraepithelial neoplasia 1 (AIN1). LSIL that sticks out from the surface of the anus or anal canal is called condyloma acuminatum.

low grade squamous intraepithelial lesion of the anus
Low grade squamous intraepithelial lesion of the anus. In this picture, numerous koilocytes can be seen in the epithelium.

Where does low grade squamous intraepithelial lesion of the anus start?

LSIL starts from squamous cells that cover the inside surface of the anus and anal canal. The squamous cells form a thin layer of tissue called the epithelium. The term “intraepithelial” means that the abnormal cells in LSIL are still located entirely within the epithelium.

Why is this condition called low grade?

LSIL is considered “low grade” because the cells look similar to the normal squamous cells found in the anus and anal canal. In contrast, the cells in high grade squamous intraepithelial lesion (HSIL) of the anus look very different than normal, healthy squamous cells.

What are the symptoms of low grade squamous intraepithelial lesion of the anus?

LSIL does not typically cause any symptoms and the condition is usually during routine screening of the anus or when the anus or anal canal are examined for another reason. However, for some patients, LSIL is associated with symptoms such as bleeding from the anus or itchiness of the anus or surrounding skin.

What causes low grade squamous intraepithelial lesion of the anus?

Most causes of LSIL are caused by long-standing infection with a low-risk subtype of human papillomavirus (HPV).

Is low grade squamous intraepithelial lesion of the anus a type of cancer?

No. LSIL is not a type of cancer. However, it is associated with a small increased risk of developing a type of anal cancer called squamous cell carcinoma over time.

What does low grade squamous intraepithelial lesion of the anus look like under the microscope?

When examined under the microscope, abnormal-looking squamous cells are located within the lower 1/3 of the epithelium on the inside surface of the anus. The term koilocyte is used to describe a large abnormal-looking squamous cell that has been infected with HPV.

What is p16 and why is it important?

Cells infected with high-risk HPV often produce large amounts of a protein called p16. Although it is much less common, cells infected with low-risk HPV, such as those in LSIL can also produce extra p16. Immunohistochemistry is a test pathologists perform to see cells producing extra p16. This test helps pathologists distinguish between LSIL and HSIL because LSIL is usually negative for p16.

Other helpful resources

Canadian Cancer Society

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