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

Aims Traditional serrated adenomas ( TSA s) 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 ( GC s): mucin‐rich TSA (Mr TSA ). The aim of this study was to determine whether this variant has unique features as compared with classic TSA ( cTSA ). Methods and results One hundred and fifty‐six TSA s were retrieved from the period 2010–2016. Patient demographics, site of polyps and 16 microscopic variables were evaluated. TSA s containing ≥50% GC s were classified as Mr TSA s. Ectopic crypt foci ( ECF s) 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 Mr TSA . In males, Mr TSA s (65%) were more prevalent than cTSA s (55%). There was no age difference, and both variants had a predilection for the left colon, although, in the right colon, Mr TSA s were more frequent (39%) than cTSA s (10%) ( P = 0.012). Adenomatous dysplasia was present in four of 24 Mr TSA s (low grade, 3; high grade, 1). The most distinctive features of Mr TSA s were: a variable growth pattern [endophytic (9%), mixed (30%), or villiform/exophytic (61%)], and a lower frequency of ECF s ( P = 0.001) and more intraepithelial lymphocytes ( P < 0.05) than in cTSA s. Mr TSA s retain characteristic luminal serrations, at least focally. Inflamed Mr TSA s can mimic inflammatory polyps and hamartomatous polyps (when there are >95% GCs). Conclusions Mr TSA is characterized by >50% GC s, and fewer ECF s 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.