Background/Purpose: Little is known about the features of T1 pancreatic ductal adenocarcinoma (PDAC) and its definition in the eighth edition of the American Joint Committee on Cancer (AJCC) staging system needs validation. The aims were to analyze the clinicopathologic features of T1 PDAC and investigate the validity of its definition. Method: Data from 1506 patients with confirmed T1 PDAC between 2000 and 2019 were collected and analyzed. The results were validated using 3092 T1 PDAC patients from the Surveillance, Epidemiology, and End Results (SEER) database. Results: The median survival duration of patients was 50 months, and the 5-year survival rate was 45.1%. R0 resection was unachievable in 10.0% of patients, the nodal metastasis rate was 40.0%, and recurrence occurred in 55.2%. The current T1 subcategorization was not feasible for PDAC, tumors with extrapancreatic extension (72.8%) had worse outcomes than those without extrapancreatic extension (median survival 107 vs. 39 months, p <.001). Extrapancreatic extension was an independent prognostic factor whereas the current T1 subcategorization was not. The results of this study were reproducible with data from the SEER database. Conclusion: Despite its small size, T1 PDAC displayed aggressive behavior warranting active local and systemic treatment. The subcategorization by the eighth edition of the AJCC staging system was not adequate for PDAC, and better subcategorization methods need to be explored. In addition, the role of extrapancreatic extension in the staging system should be reconsidered.
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