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Difficulty: Very hardPathology and Biopsy Interpretation

During a multidisciplinary gastrointestinal tumor board, the team reviews the pathology of a 15-mm sessile mucosal elevation completely excised from the right colon of a 48-year-old patient. The pathologist's report describes a mucosal proliferation characterized by a prominent saw-toothed, serrated architecture extending deep into the crypt bases. The crypt bases are notably dilated, often displaying a horizontal, 'L-shaped' or 'boot-like' growth pattern along the muscularis mucosae. Conventional nuclear stratification and elongation (cytologic dysplasia) are absent. Ancillary molecular testing of the lesional tissue demonstrates a BRAFBRAF V600E mutation and high microsatellite instability (MSI-H) due to MLH1MLH1 promoter hypermethylation.

Based on these histopathological and molecular findings, which of the following is the most accurate diagnostic interpretation?

  1. The deep serrations and crypt base dilation identify this as a sessile serrated lesion, which possesses high malignant potential via the CpG island methylator phenotype pathway.Answer
  2. B
    The complete absence of conventional cytologic dysplasia confirms a diagnosis of a hyperplastic polyp, a benign reactive condition with negligible risk of malignant transformation.
  3. C
    The presence of high microsatellite instability establishes a diagnosis of Lynch syndrome, necessitating immediate germline genetic testing for the patient and first-degree relatives.
  4. D
    The horizontal crypt growth and serrated architecture represent a traditional serrated adenoma, which typically progresses to malignancy through an APCAPC gene mutation pathway.
  5. E
    The morphological findings combined with the molecular profile indicate an inflammatory pseudopolyp that has undergone an incidental acquired mismatch repair deficiency.

Answer

The deep serrations and crypt base dilation identify this as a sessile serrated lesion, which possesses high malignant potential via the CpG island methylator phenotype pathway.
The correct answer accurately identifies the lesion as a sessile serrated lesion (SSL). The defining morphological hallmarks of an SSL include serrated architecture that extends entirely to the crypt base, accompanied by dilated, horizontally branching ('boot-shaped' or 'L-shaped') crypts. Unlike conventional tubular adenomas, SSLs typically lack traditional cytologic dysplasia (nuclear stratification and hyperchromasia). SSLs are the primary precursor lesions in the 'serrated pathway' to colorectal cancer. This pathway is driven by the CpG island methylator phenotype (CIMP), which causes widespread epigenetic silencing of genes. When CIMP leads to hypermethylation of the MLH1MLH1 mismatch repair gene promoter, it results in high microsatellite instability (MSI-H). The presence of a BRAFBRAF V600E mutation is a signature molecular event in this specific sporadic pathway.

Step-by-Step Solution

1
Analyze the histopathological description of the colonic polyp.
The lesion exhibits serrated architecture extending to the crypt bases, with dilated, 'boot-shaped' crypts and an absence of conventional cytologic dysplasia.
These specific morphological features differentiate a sessile serrated lesion (SSL) from a simple hyperplastic polyp, which only has serration in the upper crypts.
2
Evaluate the ancillary molecular testing results.
The tissue is positive for a BRAF V600E mutation and exhibits MSI-H caused by MLH1 promoter hypermethylation.
This molecular profile is the hallmark of the sporadic CpG island methylator phenotype (CIMP) pathway to colorectal cancer.
3
Differentiate the molecular findings from hereditary cancer syndromes.
The MLH1 silencing is due to acquired promoter hypermethylation, not a germline mutation.
This distinguishes sporadic MSI-H colorectal tumors arising from SSLs from hereditary Lynch syndrome, which is driven by germline mismatch repair mutations.
4
Synthesize the findings to formulate the final diagnostic interpretation.
The combination of boot-shaped crypts, right-sided location, BRAF mutation, and acquired MLH1 hypermethylation confirms a sessile serrated lesion with high malignant potential.
Accurate identification is critical because SSLs are significant premalignant precursors requiring careful endoscopic surveillance.

Key Concept

Sessile serrated lesions (SSLs) are premalignant right-sided colonic polyps characterized morphologically by deep crypt serration and dilation ('boot-shaped' crypts), and molecularly by the CIMP pathway, leading to sporadic MSI-H via acquired MLH1MLH1 hypermethylation and BRAFBRAF mutations.
Estimated Time:2m 0s
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Topics

Pathology and Biopsy Interpretation

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