Anus & perianal area


Squamous dysplasia

Topic Completed: 1 September 2014

Minor changes: 7 December 2021

Copyright: 2002-2022,, Inc.

PubMed Search: Dysplasia [title] anus "loattrfree full text"[sb]

Nicole D. Riddle, M.D.
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Cite this page: Riddle N. Squamous dysplasia. website. Accessed January 21st, 2022.
Definition / general
  • Topic discusses dysplasia of the squamous epithelium of the anus / perianal area
  • Dysplasia is defined similarly to the cervix, as increased nuclear size with hyperchromasia and irregular nuclear borders
  • Low grade squamous intraepithelial lesion:
    • AIN 1: Low grade dysplasia (the lower 1/3 of the epithelium); includes HPV change / condyloma
    • Note: Some people separate out pure HPV change from AIN I based on if there is any dysplasia at all or just HPV change; however convention dictates they both qualify for LGSIL
  • High grade squamous intraepithelial lesion:
    • The current preferred terminology though often the clinicians will requests AIN 2 or AIN 3 be specified in parenthesis
    • AIN 2: Used to be called indermediate grade; dysplasia reaches to the middle 1/3 of the epithelium
    • AIN 3: Dysplasia reaches to the upper 1/3 of the epithelium
      • Some people use this interchangeably with carcinoma in situ, others split them into two groups based on if there is any maturation or "does the top look like the bottom"
      • Note: There is some support for calling this lesion high grade squamous intraepithelial lesion (AIN 3) instead of carcinoma in situ for insurance purposes
  • Also called anal intraepithelial neoplasia (AIN), carcinoma in situ (if high grade, AIN 3)
  • Occurs in perianal skin or anal canal in flat mucosa or with condyloma acuminatum
  • Related to HPV, similar to the cervix
Clinical features
  • May be incidental finding in hemorrhoid specimen
  • Often multicentric
  • If incidental finding in hemorrhoid specimen, excision is usually curative even for high grade AIN / ACIN (Hum Pathol 1991;22:528)
Microscopic (histologic) description
  • Defined similarly to cervix: increased nuclear size with hyperchromasia and irregular nuclear borders
  • Level of dysplasia defines low grade (AIN 1) or high grade (AIN 2 - 3)
Microscopic (histologic) images

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Cytology description
  • Similar to cervical pap smears, same criteria
  • LSIL: koilocytes, nuclear enlargement 3x normal size
  • HSIL: increased nuclear cytoplasmic ratio, nuclear hyperchromasia and irregularity
Cytology images

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Positive stains
  • Similar to cervix
  • p16: should be diffuse block staining to call positive (a common mistake among pathologists is to call positive if only focal p16 staining)
  • Ki67: staining of cells above the basement membrane
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