Longitudinal prognostication in retroperitoneal sarcoma survivors: Development and external validation of two dynamic nomograms

•The individual prognosis of retroperitoneal sarcoma survivors evolves during FU.•Prognostic factors are time elapsed from surgery, event history, baseline variables.•Landmark analysis allows prognosis prediction during follow-up.•Dynamic nomograms predict individual OS and disease free survival probabilities of retroperitoneal sarcoma survivors.