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colorectal flat adenoma

Tuesday 13 March 2012

Flat or depressed adenomas are a variant of tubular adenoma with little or no mucosal elevation.

By definition, the thickness of the adenomatous mucosa does not exceed twice that of the non-adenomatous mucosa in flat adenomas.

The adenomatous changes concentrate near the luminal surface.

A disproportionate number of flat adenomas contain high-grade dysplasia (41% to 42%) as determined by a higher nuclear: cytoplasmic ratio and degree of cellularatypia than typically seen in polypoid adenomas.

They are also more likely to harbor an invasive carcinoma than their polypoid counterparts.

Flat adenomas contain crowded adenomatous tubuleswith diameters that are smaller than those seen in the more common polypoid, tubular adenomas.

This feature increases the glandular density when compared to elevated tubular adenomas.

The adenomatous tubules tend to occupy the full thickness of the lamina propria at the center ofthe lesion with superficial growth to the periphery.

Depressed adeno-mas measuring @<@1 mm in diameter show horizontal growth between the normal adjacent crypts, often leaving normal crypts entrapped as residual islands, whereas polypoid adenomas tend to grow expansively without including remnants of the normal crypts.

The mean labeling index for proliferating cells is higher indepressed adenomas than in non-depressed adenomas, but lower than seen in intramucosal carcinomas.

Flat lesions are also more commonly aneuploid, and demonstrate differential expression of many genes compared with normal mucosa or polypoid adenomas.

For example, Ras gene mutations are significantly more common in flat than polypoid adenomas,and epigenetic changes occur less frequently.

See also

- colorectal adenomas
- colorectal polyps