Low grade squamous intraepithelial lesion of the anal canal

by Jason Wasserman MD PhD FRCPC
December 18, 2023

Low grade squamous intraepithelial lesion (LSIL) in the anal canal and anus is an abnormal growth caused by infection with human papillomavirus (HPV). The abnormal growth seen in LSIL is an example of dysplasia. 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.

The virus infects squamous cells found on the surface of the anal canal and anus which over time develop genetic changes that lead to LSIL. 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. 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.

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 and anal canal?

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.

Is low grade squamous intraepithelial lesion of the anus and anal canal 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 and anal canal 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.

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have questions about this article or your pathology report. For a complete introduction to your pathology report, read this article.

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