Serrated epithelial colorectal polyps (hyperplastic polyps, sessile serrated adenomas) with perineurial stroma: Clinicopathological and molecular analysis of a new series.

Serrated colorectal fibroblastic polyps (FPs) are rare benign mucosal lesions composed of serrated epithelial crypts separated and distorted by intimately associated bland spindle cell proliferations with perineurial-like phenotype. We herein describe 21 new FPs affecting 10 females and 9 males aged 45 to 80 yrs. (mean, 62 yrs). Lesions originated in the sigmoid colon/rectosigmoid junction (n = 16), rectum (n = 2), and other parts of the colon (n = 3). Most patients had additional synchronous or metachronous polyps: classical adenomas (12 patients), sessile serrated adenoma/SSA (1 patient), hyperplastic polyps/HPs (7 patients), both HPs and adenomas (6 patients) and colorectal cancer (2 patients). Size of the lesions varied from 1 to 6 mm (mean: 3 mm). Histologically, all lesions were composed of serrated epithelial crypts that were separated and distorted by spindle cell stromal proliferations (consistently EMA+, claudin-1+ and GLUT-1+). The epithelial component displayed features of HPs (n = 17) and SSA (n = 4). Laser-microdissection-guided molecular testing was successful for 13 epithelial and 9 stromal components (9 paired samples). The BRAF V600E mutation was detected in 54% of the epithelial but in none of the stromal components. In conclusion, colorectal FPs represent genuine serrated epithelial polyps corresponding either to HP or (less frequently) SSA and should be better classified as such with a note on the presence of the stromal component. A more concise terminology reflecting their epithelial nature is needed to fulfill the requirements for colorectal cancer risk assessment and hence adopt appropriate follow-up strategies.