Objectives Misplaced epithelium in adenomas can occasionally be difficult to distinguish from invasive adenocarcinoma. We evaluated interobserver variability in the assessment of left-sided colon polypectomies for pseudoinvasion versus invasive adenocarcinoma and further investigated relevant histological findings.

Methods 28 consecutive left-sided colon polyps with the keywords “pseudoinvasion”, “epithelial misplacement”, “herniation”, “prolapse” or “invasive adenocarcinoma” were collected from 28 patients and reviewed by eight gastrointestinal pathologists. Participants assessed stromal hemosiderin, lamina propria/eosinophils surrounding glands, desmoplasia, high grade dysplasia/intramucosal adenocarcinoma and margin status and rendered a diagnosis of pseudoinvasion, invasive adenocarcinoma, or both.

Results Agreement among pathologists was substantial for desmoplasia (κ=0.70), high grade dysplasia/intramucosal adenocarcinoma (κ=0.66), invasive adenocarcinoma (κ=0.63) and adenocarcinoma at the margin (κ=0.65). There was moderate agreement for hemosiderin in stroma (κ=0.53) and prolapse/pseudoinvasion (κ=0.50). Agreement was low for lamina propria/eosinophils around glands (κ=0.12). For invasive adenocarcinoma, seven or more pathologists agreed in 24 of 28 cases (86%), and there was perfect agreement in 19/28 cases (68%). For pseudoinvasion, seven or more pathologists agreed in 19 of 28 cases (68%), and there was perfect agreement in 16/28 cases (57%).

Conclusion Moderate to substantial, though imperfect, agreement was achieved in the distinction of pseudoinvasion from invasive carcinoma.