Gastric Mixed Neuroendocrine-Non-Neuroendocrine Neoplasm: An Unusual Tumor and Its Presentation in a Young Adult

Gastric cancer is the fourth most common cause of cancer death, with more than 90% of the cases being adenocarcinomas. Among the diverse subtypes, mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN) is one of the rarest types. Gastric cancer can manifest with significant bleeding in up to 5% of patients. The authors present a case of a healthy 26-year-old male who experienced two episodes of major upper gastrointestinal bleeding that were resolved with endoscopic treatment. During the second endoscopy, a 15-mm nodular subepithelial lesion was identified at the gastroesophageal junction. Endoscopic ultrasound revealed a homogeneous and hypoechoic lesion with well-defined limits in the deep mucosa. Histological examination of the biopsies showed an adenocarcinoma. The patient later underwent a distal esophagectomy and a total gastrectomy, followed by chemotherapy. Histological examination of the surgical specimen showed a mixed adenoneuroendocrine carcinoma composed of an adenocarcinoma with tubular/glandular pattern and signet ring cells and a large cell-type neuroendocrine carcinoma. The neoplasia had infiltrated the outer muscular layers of the stomach and had disseminated to 3 regional lymph nodes, leading to its classification as stage IIb. Two years following the treatment, there is no evidence of recurrence. All genetic tests applied were negative. A MiNEN occurs when both neuroendocrine and non-neuroendocrine components represent at least 30% of the lesion. Due to its rarity, epidemiology and standard treatment are not well established because most data published are from case reports. In this context, we present a compelling case study, highlighting the patient's young age, the rarity of this specific cancer, and its uncommon presentation.