Pseudoinvasion and squamous metaplasia/morules in colorectal adenomatous polyp: a case report and literature review

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. 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. 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.