Mucin-rich variant of traditional serrated adenoma: a distinct morphological variant

Traditional serrated adenomas (TSAs) account for 5% of serrated polyps, and have a villiform architecture, eosinophilic cells with a brush border, and indented, flat-topped luminal serrations. However, some are composed of mucin-filled goblet cells (GCs): mucin-rich TSA (MrTSA). The aim of this study was to determine whether this variant has unique features as compared with classic TSA (cTSA).

One hundred and fifty-six TSAs were retrieved from the period 2010-2016. Patient demographics, site of polyps and 16 microscopic variables were evaluated. TSAs containing ≥50% GCs were classified as MrTSAs. Ectopic crypt foci (ECFs) were quantified as low (1-10) or high (>10), counted at ×200 magnification, and the average was taken for 10 fields. Twenty-four fulfilled the criteria for MrTSA. In males, MrTSAs (65%) were more prevalent than cTSAs (55%). There was no age difference, and both variants had a predilection for the left colon, although, in the right colon, MrTSAs were more frequent (39%) than cTSAs (10%) (P = 0.012). Adenomatous dysplasia was present in four of 24 MrTSAs (low grade, 3; high grade, 1). The most distinctive features of MrTSAs were: a variable growth pattern [endophytic (9%), mixed (30%), or villiform/exophytic (61%)], and a lower frequency of ECFs (P = 0.001) and more intraepithelial lymphocytes (P < 0.05) than in cTSAs. MrTSAs retain characteristic luminal serrations, at least focally. Inflamed MrTSAs can mimic inflammatory polyps and hamartomatous polyps (when there are >95% GCs).

MrTSA is characterized by >50% GCs, and fewer ECFs than cTSA, but with preservation of archetypal luminal serrations. Awareness of this variant will prevent misdiagnosis, given the association of TSA with the accelerated pathway to colorectal cancer.