Delayed Diagnosis and Rapid Progression of Colorectal Small-Cell Neuroendocrine Carcinoma

Background: Colorectal small-cell neuroendocrine carcinoma (NEC) is an uncommon, highly aggressive malignancy that can mimic benign adenomatous lesions, leading to diagnostic delay and poor prognosis. Case Presentation: A 67-year-old patient underwent colonoscopy after a positive faecal occult blood test, which revealed a large ulcerated yellowish sigmoid lesion with raised margins. Endoscopic biopsies showed only a tubulovillous adenoma with low-grade dysplasia. Because of benign histology and scheduling constraints, surgery was postponed for 40 days. Laparoscopic sigmoid colectomy with lymphadenectomy ultimately revealed a poorly differentiated small-cell NEC (pT2) with five of twelve lymph nodes positive. Immunohistochemistry showed strong CD56 and NSE positivity, CK20 negativity in tumour cells, and a Ki-67 index > 90%. Forty days post-operatively, PET-CT demonstrated new liver and lung metastases, indicating rapid systemic dissemination. Conclusion: This case underscores the diagnostic challenges of colorectal NEC and the potential impact of treatment delay. When endoscopic findings appear malignant despite benign histology, deep biopsies, expedited resection, and early staging are essential to prevent disease progression. This case emphasizes the need for timely management in lesions with discordant clinical and histopathologic features.