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Background: Submucosal pseudoinvasion and squamous metaplasia (SM) are incidental and special morphological findings in colorectal adenomas, and both can mimic invasive carcinoma. The coexistence of these two findings further increases the risk of misdiagnosis, posing a great diagnostic challenge to pathologists. From 1979 to 2022, only 8 cases have been reported, which was extremely rare. In this report, we presented a case of sigmoid colon adenoma accompanied by pseudoinvasion and SM. Additionally, relevant literature was analyzed to summarize the clinical and pathological characteristics. Case presentation: A 51-year-old Chinese male patient presented with fresh blood after defecation. Electronic colonoscopy revealed multiple polyps, which were removed using a snare and subjected to high-frequency electrocoagulation resection. The largest polyp, located in the sigmoid colon, was a thick pedunculated and lobulated polyp with a maximum diameter of 2.8 cm. The surface of the polyp showed slight ruggedness and redness, and it was sent for pathological examination. Grossly, the polyp had a lobulated and slightly rough surface. Microscopically, it showed a tubulovillous adenoma with focal high-grade dysplasia and mucosal muscle hyperplasia. Glandular elements were observed in the submucosal layer, forming a well-defined lobular structure. Some of the glands displayed cystic change, and focal SM could be seen within the adenoma. SM could manifest as discrete solid cell nests of varying sizes or cribriform-morular-like structures. Immunohistochemical staining showed that SM cells were diffusely positive for cytokeratin 5/6 (CK5/6); p40, p63, and cytokeratin 20 (CK20) were negative; while caudal type homeobox 2 (CDX2) was weakly positive. β-catenin showed abnormal nuclear expression, and an extremely low Ki67 proliferation index was observed. Conclusions: Coexistence of SM and pseudoinvasion in colorectal adenomas is highly rare. It is more commonly observed in males and tends to occur in the sigmoid colon. It primarily manifests in tubulovillous adenoma and tubular adenoma, with a majority of cases exhibiting a pedicle. Histologically, it is similar to invasive lesions. The cystic dilation of the submucosal glands, hemosiderin deposition, and the presence of a lamina propria around the submucosal glands without adjacent desmoplastic reaction, suggest pseudoinvasion rather than cancer. The bland cytological morphology and Immunohistochemical markers play a crucial role in distinguishing SM from true invasive lesions. [ABSTRACT FROM AUTHOR]

MFAP5, a 25-kD microfibril-associated glycoprotein that is involved in elastic microfibril assembly, has been demonstrated to be significantly downregulated in tumor stroma by previous gene expression study. The aim of this study was to confirm the reduced expression of MFAP5 in colonic tumor stroma using immunohistochemistry and to explore the utility of MFAP5 as a marker to facilitate diagnosing an invasive component versus pseudoinvasion in colon polyps. In all 19 colon cancer resection cases evaluated, while there was intact MFAP5 immunoreactivity in the uninvolved normal connective tissue, there was marked reduction of MFAP5 immunoreactivity in the desmoplastic stroma surrounding the invasive component. The difference in MFAP5 expression levels was most pronounced within the tumor, while a more heterogeneous expression pattern was observed at the tumor invasive front. Reduction of MFAP5 staining was also observed in the stroma around mucin pools in 6 out of 9 sections from mucinous adenocarcinomas and in areas with high-grade dysplasia. For the polypectomy cases, intact expression of MFAP5 was seen in the stroma surrounding the displaced adenomatous glands in 9 out of 12 polyps with pseudoinvasion. Loss of expression of MFAP5 was observed in the stroma surrounding small foci of invasive adenocarcinoma in 8 of 10 malignant polyps. MFAP5 is a useful marker that may help distinguish normal connective tissue from stroma within invasive colonic adenocarcinoma. MFAP5 may facilitate the distinction between pseudoinvasion and true invasive cancer in colonic adenomatous polyps with a sensitivity of 80% (confidence interval 44-96%) and a specificity of 75% (confidence interval 43-93%) in this small cohort. [ABSTRACT FROM AUTHOR]