U. Güller, R. Warschkow, C. J. Ackermann, B. M. Schmied, T. Cerny, and S. Ess, “Lower hospital volume is associated with higher mortality after oesophageal, gastric, pancreatic and rectal cancer resection”, Swiss Med Wkly, vol. 147, no. 2930, p. w14473, Jul. 2017.