Squamous metaplasia of the rectum/anus

Squamous metaplasia of the anus is a metaplastic change of the columnar epithelium. It can histologically mimic anal intraepithelial neoplasia (AIN), especially. high-grade squamous intraepithelial lesion (HSIL).

Microscopic

Features:

• Uniform cell spacing - no crowding - key feature.
• Nuclei are uniform size and round.
  • +/-Nucleoli present.
• Distinct cell borders
• +/-Intercellular bridges (due to edema) - common.

Negatives:

• No mitoses/rare basal mitoses - think cancer/AIN if you see 'em.
• Usually no hyperchromatism (think cancer/CIN if you see it).

DDx:

• Anal intraepithelial lesion.

Images

• 
  ISM - very low mag.
• 
  ISM - low mag.
• 
  ISM - intermed. mag.
• 
  ISM - high mag.

IHC

• p16 -ve
  • Usually weak-to-moderate patchy, not full thickness.
• Ki-67 - scattered cells positive.

See also

• Squamous metaplasia.