A 49-year-old woman with a family history of colorectal cancer underwent screening colonoscopy, which revealed a 35 mm non-granular laterally spreading tumor (LST-NG) in the cecal fundus. Owing to her previous appendectomy, underwater-EMR was scheduled. After resection, another lesion emerged through the appendiceal stump, prompting us to opt for full-thickness resection (FTR). Post-resection histology unexpectedly identified a high-grade neuroendocrine neoplasm (NEN) with characteristics of large-cell neuroendocrine carcinoma (LCNEN), Ki67> 60%, and pT1 in the appendiceal stump. The cecal lesion was a sessile serrated lesion with intramucosal low-grade adenocarcinoma foci. Endoscopic resection achieved R0. Right hemicolectomy was performed, and no evidence of nodal metastases was observed. After radical surgical treatment,  patient started adjuvant therapy with somatostatin analogs.