Database We used the SEER database to analyze the data of patients with PDAC. The database includes patients of 18 registries in the USA from 1973-2013, encompassing approxi-mately 28% of the USA population. All the malignant cases were followed-up annu-ally to determine vital status.
Patient Population All the variables' definitions are encoded in the SEER database. To identify the PDAC cases, site codes (C25 pancreas, C25.0-C25.9) and histology codes (8140 adenocarcinoma, 8500 infiltrating duct carcinoma) based on the International Classification of Diseases for Oncology, Third Edition (ICD-O-3) were used.11 Only cases that underwent PD and microscopically confirmed were included.
Outcome Variables We only included those PDAC patients that underwent PD with precise data available for the following variables: age at diagnosis, year of diagnosis, gender, race, tumor site, tumor size, regional nodes positive, regional nodes examined, grade, stage, vital status, and survival months.
To clearly differentiate the difference of long-term survival between young and elderly patients, all cases were divided into two group age \< 80 years and age ≥ 80 years. Since 2004, the AJCC 6th stage has been used in the SEER database. Thus, the diagnosis years of the cases included in our study ranged from 2004-2013. To analyze the median OS between different time periods, we divided the year of diagnosis into two groups: 2004-2008 year and 2009-2013. Furthermore, we also divided the tumor size into three groups: ≤ 2 cm, 2-4 cm, and \> 4 cm. The lymph node ratio (LNR) is considered a robust prognostic factor after resection of pancreatic cancer and was es-timated using regional nodes positive divided by regional nodes examined.15 LNR was then categorized into three groups: 0%, 1-50%, \> 50%.
