Diagnostic Challenges Caused by Endoscopic Biopsy of Colonic Polyps: A Systematic Evaluation of Epithelial Misplacement with Review of Problematic Polyps from the Bowel Cancer Screening Program, United Kingdom

Endoscopic mucosal biopsy may misplace mucosal elements into the submucosa of colonic adenomas, mimicking invasive adenocarcinoma. Biopsy-related misplacement can be more challenging to recognize than typical misplaced epithelium (pseudoinvasion) in pedunculated polyps. We compared the features of 16 polyps with biopsy-related misplaced epithelium with those of 10 adenomas with pseudoinvasion and 10 adenomas with invasive adenocarcinoma and performed Ki67 and p53 immunostaining on all cases. Features of misplaced epithelium in polyps referred to the Bowel Cancer Screening Program Expert Board in the United Kingdom were also evaluated for the same morphologic features. Biopsy-related epithelial misplacement occurred in adenomas throughout the colon and often appeared infiltrative (69%), including epithelial cells singly dispersed within reactive fibroinflammatory stroma or granulation tissue (44%). Misplaced epithelium displayed only low-grade cytologic features and was associated with extruded mucin (75%), tattoo pigment (63%), and misplaced normal glands (38%); scant lamina propria and muscularis mucosae were often present (88% and 44%, respectively). Cases referred to the Bowel Cancer Screening Program Expert Board also contained infiltrative-appearing misplaced epithelium (91%) that was cytologically low grade (72%), contained nondysplastic glands (11%), and showed other signs of injury. In contrast, misplaced epithelium in pedunculated polyps always had a lobular contour with a rim of lamina propria, hemorrhage, and/or hemosiderin. Invasive carcinomas showed malignant cytology and desmoplasia; most (70%) lacked features of trauma. Ki67 and p53 staining was patchy and weak in the misplaced epithelium, whereas invasive carcinomas showed increased staining for one or both markers. Pathologists should be aware that endoscopically manipulated adenomas may contain misplaced epithelium that simulates malignancy.