90% of crypts are straight and narrow but one crypt is dilated and flattened."Indeterminate polyp (SSA vs HP)" is used for serrated polyps with indeterminate or mixed features, for example:.The criteria for the recognition of SSP/A can be expected to be refined in the future.There is poor interobserver agreement in the recognition and diagnosis of SSA by both endoscopists and pathologists (Hetzel 2010).We suggest that lesions that are left sided and 0.5 cm proximal to the splenic flexure are SSA.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).Using strictly the histologic criteria above (and even genetic studies), many small left sided lesions could be considered sessile serrated adenoma.Usually large (≥1 cm) sessile right sided lesions.Sessile serrated adenoma with dysplasia if focal (see below).Traditional serrated adenoma if present throughout (see Differential Diagnosis at left), or.Presence of dysplasia in a lesion with hyperplastic features suggests either.Nuclei may be mildly atypical with open chromatin and distinct nucleoli.Nuclear stratification and loss of polarity are not seen.Significant nuclear dysplasia is not a feature of pure sessile serrated adenoma.Most crypts extend from lumen to muscularis mucosae.Lacks the architectural complexity of tubular adenomas.May be visualized with Ki67 and CK20 stains.Abnormalities of location of proliferative and mature compartments within the length of the crypt.Crypt compartmentalization aberration (CCA) (Torlakovic 2008).Patchy proliferative zones can be seen at all levels.Proliferative zone frequently displaced from base.Mature mucinous cells at base of crypts.Apical cytoplasm typically filled with microvesicular mucin.Serration is variably present throughout gland length.The majority of crypts lack the uniform pattern of prolferative bases with regular maturation towards the surface.A recent consensus conference recommends that a single abnormal crypt be sufficient for the diagnosis of SSA (Rex 2012).The WHO requires at least three adjacent abnormal crypts to make the diagnosis.How many abnormal crypts are required for the diagnosis is currently in a state of flux.Moderate dilation without flattening is suggestive but not diagnostic.“L, inverted T, hockey stick or boot” shaped crypt bases.Marked dilation of crypts with flattened, horizontal bases.Architectural disturbances of the bases of crypts is required.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.SSA appeals to those who note the genetic abnormalities and association with carcinoma.SSP appeals to those who note that the lesion lacks neoplastic cytologic features.Sessile serrated adenoma (SSA) and sessile serrated polyp (SSP) are equivalent terms.Be cautious about using this as a descriptive term even though hyperplastic polyps are serrated too.Note: many gastroenterologists use the term "serrated polyp" to mean "sessile serrated adenoma/polyp.Serrated polyp with abnormal proliferation.Cytologically bland lesion of the large intestine composed of serrated glands with architectural disturbances of the deep crypts.Sessile Serrated Polyp / Adenoma Definition
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