Sessile Serrated Polyp / Adenoma
Definition
- Cytologically bland lesion of the large intestine composed of serrated glands with architectural disturbances of the deep crypts
Alternate/Historical Names
- Serrated adenoma
- Serrated adenoma type I
- Serrated polyp with abnormal proliferation
- Note: many gastroenterologists use the term "serrated polyp" to mean "sessile serrated adenoma/polyp
- Be cautious about using this as a descriptive term even though hyperplastic polyps are serrated too
Covered separately
Background
- Sessile serrated adenoma (SSA) and sessile serrated polyp (SSP) are equivalent terms
- SSP appeals to those who note that the lesion lacks neoplastic cytologic features
- SSA appeals to those who note the genetic abnormalities and association with carcinoma
- Although not generally used as diagnostic criteria, characteristic location and genotypic abnormalities linking SSA/P to sporadic MSI high colorectal carcinoma (serrated carcinoma pathway) are the main justifications for defining this as a separate lesion from usual hyperplastic polyp
Diagnostic Criteria
- Architectural disturbances of the bases of crypts is required
- Marked dilation of crypts with flattened, horizontal bases
- “L, inverted T, hockey stick or boot” shaped crypt bases
- Moderate dilation without flattening is suggestive but not diagnostic
- Marked dilation of crypts with flattened, horizontal bases
- How many abnormal crypts are required for the diagnosis is currently in a state of flux
- The WHO requires at least three adjacent abnormal crypts to make the diagnosis
- A recent consensus conference recommends that a single abnormal crypt be sufficient for the diagnosis of SSA (Rex 2012)
- The majority of crypts lack the uniform pattern of prolferative bases with regular maturation towards the surface
- Serration is variably present throughout gland length
- Usually exaggerated serration
- Apical cytoplasm typically filled with microvesicular mucin
- Goblet cells not conspicuous
- Mature mucinous cells at base of crypts
- Serration is variably present throughout gland length
- May form serrations at base
- Proliferative zone frequently displaced from base
- Patchy proliferative zones can be seen at all levels
- Crypt compartmentalization aberration (CCA) (Torlakovic 2008)
- Abnormalities of location of proliferative and mature compartments within the length of the crypt
- Encompasses most of the above criteria
- May be visualized with Ki67 and CK20 stains
- Abnormalities of location of proliferative and mature compartments within the length of the crypt
- Lacks the architectural complexity of tubular adenomas
- Most crypts extend from lumen to muscularis mucosae
- Significant nuclear dysplasia is not a feature of pure sessile serrated adenoma
- Nuclear stratification and loss of polarity are not seen
- Nuclei may be mildly atypical with open chromatin and distinct nucleoli
- Presence of dysplasia in a lesion with hyperplastic features suggests either
- Traditional serrated adenoma if present throughout (see Differential Diagnosis at left), or
- Sessile serrated adenoma with dysplasia if focal (see below)
- Usually large (≥1 cm) sessile right sided lesions
- Using strictly the histologic criteria above (and even genetic studies), many small left sided lesions could be considered sessile serrated adenoma
- As sporadic MSI high colorectal carcinomas are unusual in the left colon, this has led to the suggestion that SSA/P should only be diagnosed on right sided lesions ≥1 cm in size (Chung)
- We suggest that lesions that are left sided and <1cm should be designated hyperplastic polyp unless they unequivocally and fully fulfill all the criteria above for SSA/P
- Left sided lesions should raise the possibility of hyperplastic polyp or traditional serrated adenoma
- Using strictly the histologic criteria above (and even genetic studies), many small left sided lesions could be considered sessile serrated adenoma
- In our experience, nearly all serrated lesions >0.5 cm proximal to the splenic flexure are SSA
- There is poor interobserver agreement in the recognition and diagnosis of SSA by both endoscopists and pathologists (Hetzel 2010)
- The criteria for the recognition of SSP/A can be expected to be refined in the future
- "Indeterminate polyp (SSA vs HP)" is used for serrated polyps with indeterminate or mixed features, for example:
- 90% of crypts are straight and narrow but one crypt is dilated and flattened
- Several of the crypts are moderately dilated but none are markedly dilated with flattening
- A <0.5 cm rectosigmoid polyp with one dilated, flattened crypt base
- A >1 cm right sided serrated polyp with only one margnally dilated and flattened gland
- Any poorly oriented specimen in which the bases of the crypts cannot be well evaluated
- (Rectosigmoid serrated polyps <0.5 cm are usually considered to be HPs even if the bases are not well seen as long as there is no clear dilation present)
- Sessile serrated adenoma may be associated with dysplasia or carcinoma
- Discrete foci of dysplasia or carcinoma may develop
- Dysplastic foci resemble classic tubular adenomas
- Many were historically called mixed hyperplastic-adenomatous polyps
- Polyps representing a true "collision" of a hyperplastic and an adenomatous polyp, especially in the right colon, must be rare
- May show usual low grade dysplasia or high grade dysplasia
- Carcinomas are frequently invasive into submucosa even when small
- Many were historically called mixed hyperplastic-adenomatous polyps
- Dysplastic foci resemble classic tubular adenomas
- Dysplastic and carcinomatous foci may be MSI high
- About half of studied cases show loss of MLH1 and PMS2
- Adjacent SSA lacks both cytologic dysplasia and MSI
- Clinical and morphologic evidence suggests that the development of dysplasia may herald rapid development of carcinoma (Sheridan 2006)
- Discrete foci of dysplasia or carcinoma may develop
- Serrated polyposis (hyperplastic polyposis)
(covered separately)
- Any of the following:
- ≥20 serrated polyps, any kind or size, anywhere in colorectum
- Some investigators prefer ≥30
- ≥5 serrated polyps proximal to sigmoid
- ≥2 of which are ≥1 cm
- Any serrated polyp proximal to sigmoid in a first degree relative of a patient with serrated polyposis
- ≥20 serrated polyps, any kind or size, anywhere in colorectum
- It is now apparent that many of the polyps of this entity are SSA
- Any of the following:
- SSP/A and hyperplastic polyps may be associated with perineurioma
or prominent submucosal fat
- No clinical significance
Robert V Rouse MD
Amirkaveh Mojtahed MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
Original posting/last update : 1/31/10, 7/15/11, 11/11/11, 6/11/12, 3/20/13, 8/19/13
