Prolapse-related changes are a confounding factor in misdiagnosis of sessile serrated adenomas in the rectum

The differential diagnosis of rectal serrated polyps is challenging due to its unique anatomic location, the evolving concept of serrated polyps over the past several years, and to histologic changes seen in rectal mucosal prolapse. We reclassified 95 rectal polyps diagnosed originally as "sessile serrated adenoma" (SSA), "serrated polyp," or "hyperplastic polyp (HP) with features of SSA" in a 5-year period based on World Health Organization classification criteria for colorectal serrated polyps. BRAF (V600E) mutation assay was performed to explore its value in the differential diagnosis for serrated polyps. Twenty-six originally diagnosed SSAs were reclassified as SSA (15/26, 57.7%), HP with mucosal prolapse (HP-P; 7/26, 26.9%), and HP (4/26, 15.4%). Fifty-two polyps originally diagnosed "HP with features of SSA" were reclassified as HP-P (24/52, 46.2%), HP (10/52, 19.2%), inflammatory-type polyp (5/52, 9.6%), and serrated polyp unclassifiable (13/52, 25.0%). Thirty-one of the 78 originally diagnosed SSA or HP with features of SSA were reclassified as HP-P, which accounted for 32.6% of the rectal polyps in this study. Mucosal prolapse along with chronic inflammation and tissue embedding artifact were the most common features that led to misdiagnosis in rectal serrated polyps. BRAF mutation was identified in 8 of 11 HP, 4 of 4 SSA, and 8 of 11 unclassifiable serrated polyp of the rectum, and was absent in control tissue. Thus, histopathologic changes suggesting prolapsed rectal mucosa should take precedence over BRAF results.