Mixed high-grade serous and large cell neuroendocrine carcinoma arising from rectal endometriosis 11 years after hysterectomy.

The malignant gastrointestinal endometriosis transformation is represented by endometriosis-associated intestinal tumors. Endometrioid adenocarcinoma and clear cell adenocarcinoma are most common among the endometrial cancers of all organs. Only four cases of mixed serous carcinoma and large cell neuroendocrine carcinoma have been reported, and all these cases originated from the uterus. A 59-year-old woman with a month's history of bloody stools was admitted. She was stable until the hematochezia occurred but is 11 years post-hysterectomy. A circumferential type-3 advanced upper rectum tumor was seen on colonoscopy. Adenocarcinoma was revealed from the forceps biopsies of the type-3 tumor component. Computed tomography showed narrowed lumen with a thickened rectum wall, a continuing mass, and a component on the anorectal side. Swollen lymph nodes were observed around the rectum, but no distant metastatic lymph nodes or organs were found. To treat the lesion, rectal surgical resection with D3 lymph node dissection was performed. Histological examination revealed combined high-grade serous and large cell neuroendocrine carcinomas. Tumor was contiguous to the endometrium in the sub-serosa. Endometriosis was determined to be the origin of both carcinomas. Therefore, endometriosis-associated intestinal tumors should be included in the differential diagnosis when rectal tumors with cystic structures are found post-hysterectomy.