Prediction models are only sparsely available for metastatic oesophagogastric cancer.\nBecause treatment in this setting is often preference-based, decision-making with the aid of a\nprediction model is wanted. The aim of this study is to construct a prediction model, called\nSOURCE, for the overall survival in patients with metastatic oesophagogastric cancer. Data from\npatients with metastatic oesophageal (n = 8010) or gastric (n = 4763) cancer diagnosed during 2005-\n2015 were retrieved from the nationwide Netherlands cancer registry. A multivariate Cox regression\nmodel was created to predict overall survival for various treatments. Predictor selection was\nperformed via the Akaike Information Criterion and a Delphi consensus among experts in palliative\noesophagogastric cancer. Validation was performed according to a temporal internal-external\nscheme. The predictive quality was assessed with the concordance-index (c-index) and calibration.\nThe model c-indices showed consistent discriminative ability during validation: 0.71 for\noesophageal cancer and 0.68 for gastric cancer. The calibration showed an average slope of 1.0 and\nintercept of 0.0 for both tumour locations, indicating a close agreement between predicted and\nobserved survival. With a fair c-index and good calibration, SOURCE provides a solid foundation\nfor further investigation in clinical practice to determine its added value in shared decision making.
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