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Background: The natural history of branch-duct intraductal papillary mucinous cystic neoplasms (BD-IPMNs) in the pancreas remains unclear. This study aimed to answer this clinical question by focusing on the development of concomitant pancreatic ductal adenocarcinomas (cPDAC)., Methods: The Japan Pancreas Society conducted a prospective multicenter surveillance study of BD-IPMN every six months for five years. The primary endpoints were progression of BD-IPMN, progression to high-grade dysplasia/invasive carcinoma (HGD/IC), and cPDAC. Factors predicting the progression of BD-IPMN to HGD/IC and development of cPDAC were also assessed as secondary endpoints., Results: Among the 2104 non-operated patients, 348 (16.5 %) showed progression of primary BD-IPMN. Cumulative incidences of BD-IPMN with HGD/IC and cPDAC during the 5.17-year surveillance period were 1.90 % and 2.11 %, respectively, and standard incidence ratios of BD-IPMN with HGD/IC and cPDAC were 5.28 and 5.73, respectively. Of 38 cPDACs diagnosed during surveillance, 25 (65.8 %) were resectable. The significant predictive characteristics of BD-IPMN for progression to HGD/IC were larger cyst size (p = 0.03), larger main pancreatic duct size (p < 0.01), and mural nodules (p = 0.02). Significant predictive characteristics for the development of cPDAC were male sex (p = 0.03) and older age (p = 0.02), while the size of IPMN was not significant., Conclusion: Careful attention should be given to "dual carcinogenesis" during BD-IPMN surveillance, indicating the progression of BD-IPMN to HGD/IC and development of cPDAC distinct from BD-IPMN, although the establishment of risk factors that predict cPDAC development remains a challenge (UMIN000007349)., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)