Case Report: Rare case of primary rectal adenocarcinoma presenting as a smooth submucosal elevation: endoscopic submucosal excavation and 4 years and 11 months follow-up.

Primary adenocarcinoma of the rectum presenting as a smooth submucosal elevation is clinically rare. Due to its insidious symptoms and diagnostic challenges in imaging differentiation, the malignant risk is easily overlooked. We report a 64-year-old female patient who was incidentally found to have a 0.9 cm diameter submucosal elevation in the rectum (15 cm from the anus) during physical examination. Endoscopic ultrasonography (EUS) revealed a hypoechoic mass originating from the muscularis mucosae. Based on intraoperative ultrasound examination, a preliminary diagnosis of rectal neuroendocrine tumor (NET) is made. The lesion was completely resected via endoscopic submucosal excavation (ESE), and postoperative pathology confirmed adenocarcinoma with negative resection margins. The patient had an uneventful postoperative recovery and was followed up for 4 years and 11 months, nearly 5 years without tumor recurrence, metastasis, or impairment of intestinal function, maintaining a good quality of life. This case highlights the necessity of vigilant evaluation for smooth submucosal rectal lesions to avoid missed diagnosis of malignancy. As an advanced minimally invasive endoscopic technique, ESE enables complete resection of submucosal lesions with the advantages of minimal trauma, rapid recovery, preserved intestinal function, and favorable long-term safety, providing valuable clinical experience for the management of similar rare cases.