Squamous metaplasia of the rectum/anus

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Squamous metaplasia of the rectum/anus is a metaplastic change of the columnar epithelium of the rectum.

It can histologically mimic anal intraepithelial neoplasia (AIN), especially. high-grade squamous intraepithelial lesion (HSIL).

General

  • Present in the transformation zone between the columnar epithelium of the rectum and squamous epithelium of the anus - similar to the transformation zone of the uterine cervix.[1]

Anatomy & histology:

  • Typically, the squamocolumnar junction is just above the dentate line (also pectinate line).
  • Squamous epithelium 1 cm proximal to the dentate line is considered abnormal.[2]
  • The dentate line divides the anal canal into upper two thirds and lower third.

Microscopic

Features:

  • Uniform cell spacing - no crowding - key feature.
  • Nuclei are uniform size and round.
  • 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:

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

References

  1. ↑ Pineda, CE.; Welton, ML. (May 2009). "Management of anal squamous intraepithelial lesions.". Clin Colon Rectal Surg 22 (2): 94-101. doi:10.1055/s-0029-1223840. PMID 20436833.
  2. ↑ Bujanda, L.; Iriondo, C.; Muñoz, C.; Etxezarraga, C.; Ramírez, MM.; Ramos, F.; Sánchez, A. (Feb 2001). "Squamous metaplasia of the rectum and sigmoid colon.". Gastrointest Endosc 53 (2): 255-6. PMID 11174313.
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