A case of a rectal adenocarcinoma mimicking a neuroendocrine tumor in the background mucosa of amoebic colitis.

A 54-year-old man with diarrhea and hematochezia for 2 months was referred to our department. A total colonoscopy revealed amoebic colitis caused by Entamoeba histolytica. Concurrently, a submucosal tumor-like yellowish hemispheric polypoid lesion was incidentally detected in the rectum. We speculated that the lesion was a NET, which could be cured by endoscopic treatment. However, histopathological assessment of a biopsy specimen unexpectedly revealed a well- or moderately differentiated adenocarcinoma. After treatment of the amoebic colitis, endoscopic ultrasound revealed a low, hetero-echoic, 6-mm-diameter lesion mainly in the submucosal layer. We performed surgical resection because the invasion was estimated to be to the deeper submucosal layer. Histopathological assessment of the surgically resected specimen revealed a focal lesion of a well-differentiated adenocarcinoma in the granulation tissue of the submucosal layer. In cases accompanied by amoebic colitis, a tumor's initial gross type might change. Diagnostic endoscopic resection could be acceptable in such cases.