Pathological Complete Response Achieved with Colorectal Cancer-Based Chemotherapy for Locally Recurrent Cecal Neuroendocrine Carcinoma after Surgery: A Case Report.

[INTRODUCTION] Primary neuroendocrine carcinoma (NEC) of the colon is extremely rare, accounting for only 0.2% of all colorectal malignancies, and is associated with a poor prognosis. Early diagnosis is often challenging, as endoscopic biopsies are frequently misinterpreted as adenocarcinoma. Although platinum-based regimens such as etoposide plus cisplatin or irinotecan are commonly used, no standard chemotherapy protocol has been established. We report a case of locally recurrent cecal NEC that responded remarkably to a colorectal cancer-based chemotherapy regimen, achieving a pathological complete response.

[CASE PRESENTATION] A 52-year-old man presented with severe anemia. Imaging and colonoscopy revealed a cecal tumor initially diagnosed as adenocarcinoma. He underwent laparoscopic right hemicolectomy with lymph node dissection. Final pathology revealed NEC, staged as pT3N2aM0, Stage IIIB. Adjuvant etoposide-cisplatin chemotherapy was initiated. Three months postoperatively, carcinoembryonic antigen (CEA) rose to 44.4 ng/mL, and CT demonstrated a perianastomotic peritoneal nodule and lymphadenopathy, consistent with recurrence. Considering the adenocarcinoma component in the primary tumor and elevated CEA, chemotherapy was switched to a colorectal cancer-based regimen: FOLFOXIRI plus bevacizumab. After 7 cycles, both radiologic regression and normalization of CEA levels were achieved. Resection of the recurrent lesions confirmed a pathological complete response with no residual tumor cells.

[CONCLUSIONS] To the best of our knowledge, this is the first reported case of cecal NEC achieving pathological complete regression with a colorectal cancer-based chemotherapy regimen. Our findings indicate that colorectal NEC may respond not only to platinum-based regimens but also to colorectal cancer-based regimens. Furthermore, CEA levels may serve as a clinically relevant biomarker to guide chemotherapy selection in this setting.