Sessile serrated adenoma

Sessile serrated adenoma, abbreviated SSA, is a premalignant polyp of the large bowel. It is also known as sessile serrated polyp (abbreviated SSP), sessile serrated lesion and sessile serrated adenoma/polyp (abbreviated SSA/P).

Sessile serrated adenoma
Diagnosis in short
SSA. H&E stain.
LM	serrated epithelium, crypt base dilation, crypt branching, boot-shaped glands, horizontal glands
LM DDx	hyperplastic polyp, tubular adenoma (when with dysplasia)
Site	colon - usually cecum or ascending colon
Associated Dx	colorectal adenocarcinoma, hyperplastic polyp
Syndromes	serrated polyposis syndrome, MUTYH polyposis syndrome
Prevalence	common
Endoscopy	flat, usually > 5 mm, mucinous cap
Clin. DDx	normal, hyperplastic polyp

General

• Colonic lesion.
• May be seen in the context of serrated polyposis syndrome.

Epidemiology:

• Thought to lead to colorectal cancer through a different pathway than most tumours in the left colon/rectum.
• Microvesicular hyperplastic polyps are hypothesized to be the the precursor of SSAs.

Gross

Features:

• Flat lesions, usually > 5 mm.
• Typically have a "mucous cap" - present ~65% of the time; useful for identification.
• Border not well-demarcated.
• More common in the proximal colon.

Note:

• Sessile lesions over 1 cm are usually SSAs.

Image:

• SSA - endoscopy (nature.com).

Microscopic

Features:

• Serrated epithelium at the surface and deep in the crypts.
  • Saw-tooth appearance, epithelium has jagged appearing edge.
• Crypt dilation at base with serrations - key feature.
  • Very common -- anecdotally the most sensitive feature.
• "Boot"-shape or "L"-shaped glands.
  • Shape may be similar to a hockey stick.
• Horizontal crypts = crypt long axis parallel to the muscularis mucosae.
• Crypt branching.

Minimal extent criteria - number of abnormal crypts with the above features:

• German Society of Pathology proposal: at least two abnormal crypts -- crypts do not have to be adjacent.
  • Onlinepathology prefers this definition.
• An expert panel lead by Rex states that one unequivocally altered crypt should prompt calling SSA.
• The WHO requires - depending on what you read:
  • Three adjacent crypts to be abnormal.
  • Two or three adjacent crypts to be abnormal.

Notes:

• Typically do not have nuclear atypia, i.e. no nuclear crowding, no nuclear hyperchromasia, no cigar-shaped nuclei.
  • SSAs with nuclear atypia may be referred to as advanced sessile serrated adenomas.

DDx

• Hyperplastic polyp.
• Tubular adenoma - for SSA with dysplasia, TAs often less than 1 cm (uncommon for SSAs).

Images

• 

  SSA - low mag. (WC/Nephron)

• 

  SSA - intermed. mag. (WC/Nephron)

• 

  SSA - high mag. (WC/Nephron)

• 

  SSA - low mag. (WC/Nephron)

• 

  SSA - intermed. mag. (WC/Nephron)

• 

  SSA - very high mag. (WC/Nephron)

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COLONIC POLYP, ASCENDING COLON, BIOPSY: 
- SESSILE SERRATED ADENOMA.
- NEGATIVE FOR DYSPLASIA.

COLONIC POLYP, ASCENDING COLON, BIOPSY: 
- SESSILE SERRATED ADENOMA WITH DYSPLASIA.

Note:

• The above exactly mirrors the Canadian consensus.

Micro

The section shows a small polypoid fragment of colonic mucosa with a serrated epithelium that focally extends to the crypt base. Several dilated crypt bases are seen. One horizontal crypt and one boot-shaped crypt are present. The epithelium matures to the surface. A small amount of submucosa is present and contains a benign lymphoid aggregate.