Rectal mixed adenoma-neuroendocrine lesion: a rare case report

Abstract 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.