Distinct histopathological characteristics in colorectal submucosal invasive carcinoma arising in sessile serrated adenoma/polyp and conventional tubular adenoma.

The histopathological characteristics of colorectal submucosal invasive carcinoma arising in sessile serrated adenoma/polyp (SSA/P), a rare malignant tumour, have not yet been fully elucidated. To investigate the features of such, we retrospectively analysed 40 submucosal invasive carcinomas with SSA/P (CA-SSA/P) and compared them to 129 cases of submucosal invasive carcinoma with conventional tubular adenoma (CA-AD). We additionally performed hMLH1 immunostaining. CA-SSA/Ps were significantly smaller than CA-ADs (P < 0.001). Histologically, well to moderately differentiated adenocarcinoma was predominant in both CA-SSA/Ps and CA-ADs. No significant differences in depth of invasion were found between the two groups. However, lymphatic invasion was more often found in CA-SSA/Ps (30%) than in CA-ADs (13%; P = 0.028), as was lymph node metastasis (CA-SSA/Ps, 28%; CA-ADs, 7%; P = 0.011). Furthermore, mucinous component and serrated architecture were significantly more frequent in CA-SSA/Ps (30 and 63%) than in CA-ADs (5 and 18%; P < 0.001, respectively). Tumour-infiltrating lymphocytes and Crohn-like inflammatory reaction were also more frequently found in CA-SSA/Ps (70 and 30%) than in CA-ADs (31 and 9%; P ≤ 0.001, respectively), whereas the opposite was true of desmoplastic reaction (CA-SSA/Ps, 35%; CA-ADs, 67%; P < 0.001). Loss of hMLH1 expression was more frequent in CA-SSA/P cases (93%) than in CA-AD cases (5%; P < 0.001). In conclusion, CA-SSA/P lesions exhibit a higher potential for lymphatic invasion and lymph node metastasis and have distinct histopathological features, including mucinous component, serrated architecture, tumour-infiltrating lymphocytes, Crohn-like inflammatory reaction, and absence of desmoplastic reaction, compared to their conventional counterparts.