Supplementary Material for: Gastrointestinal neuroendocrine neoplasms in children and adolescents – data from the German MET studies (1997-2024)

Introduction: Gastrointestinal neuroendocrine neoplasms (GI-NENs) outside the appendix and pancreas are exceptionally rare in children and adolescents. Limited data on presentation, treatment, and outcomes hinder clinical decision-making. Methods: We retrospectively analyzed 16 patients under 18 years with histologically confirmed NENs of gastrointestinal origin, enrolled in the German Malignant Endocrine Tumor (MET) Registry from 1997 to 2024. Neuroendocrine carcinomas (NECs) were eligible for inclusion but were not observed. Findings were compared with a cohort (age 0-20 years) from the SEER database. Results: Median age at diagnosis was 15.4 years; 62.5% were male. Primary tumor sites included the stomach (43.8%), colorectum (18.8%), duodenum and Meckel´s diverticulum (12.5% each), and jejunum and omentum majus (6.3% each). Distant metastases were present in 31.3%, with no isolated lymph node involvement. was All tumors were well-differentiated NETs: G1 (43.8%), G2 (37.5%), and G3 (6.3%). Hereditary syndromes were confirmed in 18.8% and suspected in 12.5%. Somatostatin receptor 2 (SSTR2) expression was seen in most tested tumors. At 30.1 months median follow-up, 3-year overall survival (OS) and event-free survival (EFS) were 93.3% and 73.3%, respectively, both associated with tumor grade, stage, and resection. The SEER cohort (n = 83) primarily had rectal primaries, localized disease, and a 3-year OS was 95.2%. Conclusion: Pediatric GI-NENs may present with advanced disease but have favorable outcomes following resection. Given the rarity and complexity, close evaluation by multidisciplinary tumor boards at each treatment step is strongly recommended to support individualized care.