Rectal mixed adenoma-neuroendocrine lesion: a rare case report

Objectives: Here we report a rare case of a rectal mixed adenoma-neuroendocrine lesion in which endoscopic submucosal dissection (ESD) proved essential for diagnosis by uncovering an unexpected high-grade neuroendocrine component within a colorectal adenoma. Case presentation: A 64-year-old woman underwent a screening colonoscopy following a positive fecal occult blood test. Endoscopic evaluation identified a 25 × 20 mm laterally spreading tumor in the distal rectum. ESD was successfully performed, achieving complete en bloc resection. Histological analysis demonstrated a tubular adenoma with high-grade dysplasia containing a large-cell neuroendocrine carcinoma (NEC) counterpart, consistent with a mixed adenona-neuroendocrine lesion. Despite R0 resection and disease staging proving a localized neoplasm (T1N0M0), since lymphatic invasion and the presence of an NEC component were identified, the patient was referred for surgical resection. Conclusions: This case highlights the diagnostic value of ESD in rectal lesions, not only as a therapeutic approach but also as a critical tool for achieving optimal specimen quality and identifying unexpected aggressive histology with major implications for staging and subsequent management.