EP220 - Mixed Neuroendocrine-Non Neuroendocrine Neoplasm of the Rectum Presenting as Attenuated Polyposis

Purpose/Background: Mixed neuroendocrine-non neuroendocrine neoplasms are rare tumors found most often in the gastro-entero-pancreatic tract. These neoplasms rarely present in patients who phenotypically have an attenuated polyposis pattern seen on endoscopy. Efforts to establish molecular characterization have found mixed neuroendocrine-non neuroendocrine neoplasms to have some genetic similarities to colorectal adenocarcinomas. Here, we present a case of a mixed neuroendocrine-non neuroendocrine in the setting of attenuated polyposis.

Methods/Interventions: This case report describes a 42-year-old male who presented with a 10-year history of rectal bleeding. Endoscopy revealed numerous colon and rectal polyps, including a rectosigmoid mass and a distal rectal mass. A CT scan of the abdomen and pelvis showed a large, lobulated, enhancing rectosigmoid mass suspicious for adenocarcinoma, with no evidence of metastasis. Subsequent flexible sigmoidoscopy identified an exophytic, fixed, friable, malignant-appearing mass at the dentate line, extending 8 cm proximally. Biopsies taken from this mass returned as tubulovillous adenoma with patchy high-grade dysplasia. Given concern for underlying malignancy, the patient underwent total proctocolectomy with permanent end ileostomy.

Results/Outcomes: Pathology of the large bowel and rectum revealed a mixed neuroendocrine-non neuroendocrine neoplasm arising from a traditional serrated adenoma, and an adenocarcinoma arising from a separate traditional serrated adenoma. One of thirty-three lymph nodes was positive for metastatic carcinoma. A mismatch repair panel showed intact MLH1, PMS2, MSH2, and MSH6, consistent with a microsatellite-stable tumor. Patient had an uneventful postoperative course and is completing adjuvant therapy.

Conclusion/Discussion: Currently, no standard guidelines exist for the classification or treatment of mixed neuroendocrine-non neuroendocrine neoplasms of the colon or rectum. Treatment is often directed at the most aggressive component of the neoplasm. Further research is needed to better understand this disease process and guide management.

Colon and Rectum Neoplasms
This is a gross photo of the distal rectum and sigmoid masses.