The two acronyms used to describe anal lesions are HSIL and LSIL. HSIL is an acronym for high grade squamous intraepithelial lesion, and LSIL means low grade squamous intraepithelial lesion. A lesion is an area of abnormal tissue, and high grade versus low grade refers to the likelihood that it will progress to cancer. HSIL may also be referred to as anal dysplasia.
What "squamous intraepithelial lesion" means is that it is an area of abnormal tissue on top of the skin. If the abnormality spreads down into the layers of tissue below the skin, that is when it is considered anal cancer. It's important to remember that very few areas of HSIL ever turn into anal cancer. It's estimated that only 1 in 1000 high grade lesions turn into cancer each year.
Lesions are generally found by a clinician using HRA, high resolution anoscopy. Whether the lesion is high grade or low grade is determined by taking a tiny biopsy of the lesion. Some lesions, especially LSIL, spontaneously resolve on their own without treatment. Anal lesions are generally painless and most people are unaware they have them until they have an HRA. Sometimes internal anal lesions may bleed and those outside the anal canal can itch. Clinicians focus on finding HSIL in particular because it is thought that treating HSIL helps prevent the lesions from becoming cancerous (although studies are still underway to confirm this).