Sometimes an invagination in a colonic adenomatous polyp seen in a histologic section may closely mimic carcinomatous invasion into the submucosa.
(1) adenomatous polyp with some degree of epithelial dysplasia
(2) often >= 1 cm in diameter with a stalk
(3) an inward invagination in the epithelial layer, possibly due to fibrosis in the submucosa following torsion, a biopsy or other trauma
Histologic features of a colonic adenoma showing pseudoinvasion:
(1) serial sections show continuity of the glands in the focus with the surface epithelium
(2) the cells in the glands are not cytologically malignant
(3) the stroma around the glands do not show desmoplasia
(4) the stroma around the glands may show hemosiderin deposits
(5) lamina propria is present around the glands
(6) the muscularis mucosae is present, thickened and disorganized
(7) the focus may contain a mixture of both normal and adenomatous glands (in true invasion only malignant glands would be seen in the submucosa)
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