br Conflicts of interest br The authors declare
Conflicts of interest
The authors declare no competing interests.
Availability of data and materials
The data of patients used in this study are available from the corresponding author upon reasonable request.
Consent for publication
Ethics approval and consent to participate
This study was approved by the ethics committee of Peking Union Medical College Hospital. All included patients signed an informed consent form voluntarily.
The authors thank all patients who participated in this study.
Appendix A. Supplementary data
 Neoptolemos JP, Stocken DD, Bassi C, Ghaneh P, Cunningham D, Goldstein D, Padbury R, Moore MJ, Gallinger S, Mariette C, et al. Adjuvant chemotherapy with fluorouracil plus folinic SB-203580 vs gemcitabine following pancreatic cancer resection: a randomized controlled trial. J Am Med Assoc 2010;304(10): 1073e81.
 Neoptolemos JP, Palmer DH, Ghaneh P, Psarelli EE, Valle JW, Halloran CM, Faluyi O, O'Reilly DA, Cunningham D, Wadsley J, et al. Comparison of adjuvant gemcitabine and capecitabine with gemcitabine monotherapy in patients with resected pancreatic cancer (ESPAC-4): a multicentre, open-label, rand-omised, phase 3 trial. Lancet 2017;389(10073):1011e24.
 Neoptolemos JP, Stocken DD, Friess H, Bassi C, Dunn JA, Hickey H, Beger H, Fernandez-Cruz L, Dervenis C, Lacaine F, et al. A randomized trial of chemo-radiotherapy and chemotherapy after resection of pancreatic cancer. N Engl J Med 2004;350(12):1200e10.
 Oettle H, Neuhaus P, Hochhaus A, Hartmann JT, Gellert K, Ridwelski K, Niedergethmann M, Zulke C, Fahlke J, Arning MB, et al. Adjuvant chemo-therapy with gemcitabine and long-term outcomes among patients with resected pancreatic cancer: the CONKO-001 randomized trial. J Am Med Assoc 2013;310(14):1473e81.
 Stemmler J, Stieber P, Szymala AM, Schalhorn A, Schermuly MM, Wilkowski R, Helmberger T, Lamerz R, Stoffregen C, Niebler K, et al. Are serial CA 19-9 ki-netics helpful in predicting survival in patients with advanced or metastatic pancreatic cancer treated with gemcitabine and cisplatin? Onkologie 2003;26(5):462e7.
 Pelzer U, Hilbig A, Sinn M, Stieler J, Bahra M, Dorken B, Riess H. Value of carbohydrate antigen 19-9 in predicting response and therapy control in patients with metastatic pancreatic cancer undergoing first-line therapy. Front Oncol 2013;3:155.
 Ko AH, Hwang J, Venook AP, Abbruzzese JL, Bergsland EK, Tempero MA. Serum CA19-9 response as a surrogate for clinical outcome in patients receiving fixed-dose rate gemcitabine for advanced pancreatic cancer. Br J Canc 2005;93(2):195e9.
 Bauer TM, El-Rayes BF, Li X, Hammad N, Philip PA, Shields AF, Zalupski MM, Bekaii-Saab T. Carbohydrate antigen 19-9 is a prognostic and predictive biomarker in patients with advanced pancreatic cancer who receive gemcitabine-containing chemotherapy: a pooled analysis of 6 prospective trials. Cancer 2013;119(2):285e92.
 Berger AC, Garcia Jr M, Hoffman JP, Regine WF, Abrams RA, Safran H, Konski A, Benson 3rd AB, MacDonald J, Willett CG. Postresection CA 19-9 predicts
overall survival in patients with pancreatic cancer treated with adjuvant chemoradiation: a prospective validation by RTOG 9704. J Clin Oncol 2008;26(36):5918e22.
 Daamen LA, Groot VP, Heerkens HD, Intven MPW, van Santvoort HC, Molenaar IQ. Systematic review on the role of serum tumor markers in the detection of recurrent pancreatic cancer. HPB (Oxford) 2018;20(4):297e304.
 Rieser CJ, Zenati M, Hamad A, Al Abbas AI, Bahary N, Zureikat AH, Zeh 3rd HJ, Hogg ME. CA19-9 on postoperative surveillance in pancreatic ductal adeno-carcinoma: predicting recurrence and changing prognosis over time. Ann Surg Oncol 2018.
 Tempero MA, Uchida E, Takasaki H, Burnett DA, Steplewski Z, Pour PM. Relationship of carbohydrate antigen 19-9 and Lewis antigens in pancreatic-cancer. Cancer Res 1987;47(20):5501e3.  Edge SB. American Joint Committee on Cancer.: AJCC cancer staging manual.
 Chiorean EG, Von Hoff DD, Reni M, Arena FP, Infante JR, Bathini VG, Wood TE, Mainwaring PN, Muldoon RT, Clingan PR, et al. CA19-9 decrease at 8 weeks as a predictor of overall survival in a randomized phase III trial (MPACT) of weekly nab-paclitaxel plus gemcitabine versus gemcitabine alone in patients with metastatic pancreatic cancer. Ann Oncol 2016;27(4):654e60.
 Halm U, Schumann T, Schiefke I, Witzigmann H, Mossner J, Keim V. Decrease of CA 19-9 during chemotherapy with gemcitabine predicts survival time in patients with advanced pancreatic cancer. Br J Canc 2000;82(5):1013e6.  Tzeng CW, Fleming JB, Lee JE, Wang X, Pisters PW, Vauthey JN, Varadhachary G, Wolff RA, Katz MH. Yield of clinical and radiographic sur-veillance in patients with resected pancreatic adenocarcinoma following multimodal therapy. HPB (Oxford) 2012;14(6):365e72.