Metastasis of endometrial adenocarcinoma masquerading as a primary rectal cancer: A rare case report with literature review

RATIONALE: The majority of rectal malignancies are primary tumors, secondary tumors are unusual. The rectal metastasis of endometrial carcinoma is reported to be extremely rare, especially in the absence of endometriosis. PATIENT CONCERNS: Herein we present a rare case of a 68-year-old postmenopausal woman with a history of endometrial adenocarcinoma, metastasizing to the rectum 5 years after a hysterectomy and bilateral salpingo-oophorectomy treatments with pelvic lymphadenectomy were performed. DIAGNOSES: Histological examination of the rectal neoplasm revealed an invasive lesion in submucosal and muscular layers without definitely invaded evidence in the serous membrane and there was also no obvious endometriosis. The results of immunohistochemistry showed the cancer cells were positive for CK7, estrogen receptor, progesterone receptor, and negative for CK20, villin, confirming the diagnosis of metastatic rectal adenocarcinoma originating from uterine endometrial adenocarcinoma. Meanwhile, the results of immunohistochemical staining showed positive expression of MSH2, MSH6, and negative expression of MLH1 and PMS2, hinting at microsatellite instability which may be related to Lynch syndrome. INTERVENTIONS: The Dixon operation with lymph node dissection was performed. Chemotherapy was also performed on this patient for the next 6 months. OUTCOMES: The patient was followed up for the next 6 months after surgery and no recurrence was documented until now. LESSONS SUBSECTIONS: Though rectal and endometrial adenocarcinoma could share some similar morphologic features, different immunohistochemical profiles could be revealed between them. Most endometrial carcinoma in the colon or rectum develop from endometriosis. Secondary rectal cancer with endometrial origination in the absence of endometriosis and serosal implants was extremely rare. Therefore, we should pay more attention to this rare but possible presentation for appropriate diagnosis and treatment of these patients.