Sessile serrated adenoma

Sessile serrated adenoma, abbreviated SSA, is a premalignant polyp of the large bowel.

Sessile serrated adenoma
Diagnosis in short
SSA. H&E stain.
Synonyms	sessile serrated lesion, sessile serrated polyp, sessile serrated adenoma/polyp
LM	serrated epithelium, crypt base dilation, crypt branching, boot-shaped glands, horizontal glands
LM DDx	hyperplastic polyp, tubular adenoma when with dysplasia, mucosal prolapse for left sided lesions or background of diverticulosis
IHC	Chromogranin A (completely) -ve
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, other intestinal polyps

It is also known as sessile serrated polyp (abbreviated SSP), sessile serrated lesion and sessile serrated adenoma/polyp (abbreviated SSA/P). In the United Kingdom, this entity and is known as a sessile serrated lesion, a terminology that is likely to be adopted in the 2019/5th edition WHO Blue Book.

This lesion should not be confused with the traditional serrated adenoma, previously known as serrated adenoma.

General

• Colonic lesion.
• May be seen in the context of serrated polyposis syndrome.
• Approximately 5% of SSAs have dysplasia.

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.
• Submucosal lipoma or pseudolipoma is often seen in associated with SSA.^{[citation needed]}
• Perineuriomas are also seen in a small proportion of cases

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.
• An expert panel lead by Rex states that one unequivocally altered crypt should prompt calling SSA.
• The 4th edition of the WHO blue book requires - depending on what you read:
  • Three adjacent crypts to be abnormal.
  • Two or three adjacent crypts to be abnormal.
  • The 5th edition is likely to make a single crypt sufficient for diagnosis.

Dysplasia

Sessile serrated adenomas typically lack "conventional" nuclear atypia, as seen in adenomata in the tubulovillous spectrum. They are nonetheless neoplastic lesions on account of architectural "dysplasia". Additionally, dysplasia may manifest in more than one way:

Intestinal or "cytological" dysplasia

As seen in conventional adenomata, i.e. nuclear hyperchromasia and crowding. SSAs with nuclear atypia may be referred to as advanced sessile serrated adenomas

Serrated dysplasia

Round nuclei, prominent nucleoli and eosinophilic cytoplasm

Minimal deviation dysplasia

As the name suggests, there is only minor architectural and cytological changes. These areas are associated with loss of MLH1 immunostaining.

DDx

• Hyperplastic polyp.
• Tubular adenoma - for SSA with dysplasia, TAs often less than 1 cm (uncommon for SSAs).
• Mucosal prolapse - especially for left sided lesions and a background of diverticulosis.

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)

IHC

• Chromogranin A -ve; complete loss of staining.
  • Normal colorectal mucosa has scattered Chromogranin A-positive cells.
  • Hyperplastic polyp has increased scattered Chromogranin A-positive cells.

Sign out

Polyp, Hepatic Flexure of Colon, Polypectomy or Biopsy: 
	- Sessile serrated adenoma, NEGATIVE for conventional adenomatous dysplasia.

Block letters

POLYP, CECUM, POLYPECTOMY: 
- SESSILE SERRATED ADENOMA.
-- NEGATIVE FOR DYSPLASIA.

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

POLYP, HEPATIC FLEXURE OF COLON, POLYPECTOMY: 
- SESSILE SERRATED ADENOMA.
-- NEGATIVE FOR DYSPLASIA.

Dysplasia present

Polyp, Ascending Colon, Polypectomy or Biopsy:
     - Sessile serrated adenoma with low-grade dysplasia, see comment.

Comment:
Sessile serrated adenomas with dysplasia are considered to be advanced lesions that 
have an increased propensity to transform to adenocarcinoma. Complete endoscopic removal is recommended. If complete endoscopic removal cannot be achieved, short-term re-endoscopy and biopsy, or surgical resection should be considered.

Block letters

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

The above mirrors the Canadian consensus.

Sign out comment

The Canadian consensus also advocates use of a comment, like the following statement:

Sessile serrated adenomas with dysplasia are considered to be advanced lesions that 
have an increased propensity to transform to adenocarcinoma. Complete endoscopic removal 
is recommended. If complete endoscopic removal cannot be achieved, short-term re-endoscopy
and biopsy, or surgical resection should be considered.

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.

See also

• Colorectal polyps.