Epithelial-Stromal Polyp With Features of Perineurioma Lacking Covering Serrated Crypts Associated With a Sessile Serrated Lesion With High-Grade Dysplasia of the Colon: A Case Report

A 79-year-old woman received a colonoscopy, and 6 polyps were found in the proximal colon. Histologically, four polyps were conventional tubular adenomas. Interestingly, one protuberant polyp was a sessile serrated lesion (SSL) with high-grade dysplasia being regarded as a potent precursor lesion for colorectal cancers. The polyp was connected with a flat-type SSL. Furthermore, another polyp was a bland spindle cell lesion filling the lamina propria. The polyp lacked covering serrated epithelium. Although the mesenchymal neoplasm displayed architectural features of perineurioma, immunoexpression of epithelial membrane antigen (EMA) and GLUT1 was negative. Accordingly, the benign mesenchymal neoplasm was considered an unusual epithelial-stromal polyp with a perineurioma-like lesion. In cases of usual perineuriomas, stromal cell proliferation is suggested to be a concern for serrated crypts that often harbor BRAF mutation. However, the mechanical nature of the proliferation of stromal cells in the polyps without serrated crypts is unknown. For the present case, proliferation of the mesenchymal cells of the epithelial-stromal polyp was suspected to relate to the serrated lesion near the polyp. Moreover, it is also presumed that the occurrence of the six polyps, including the serrated lesion, mesenchymal neoplasm, and tubular adenomas in the proximal colon, took part in both the BRAF and WNT signal pathways.