Prediction-model development and temporal validation for six-month cancer-specific mortality in

Background: Methods: Patients diagnosed in 2010-2021 were identified from the Surveillance, Epidemiology, and End Results (SEER) database. Eligible cases had C16.0 cardia primary site, adenocarcinoma-related histology, malignant behavior, positive histology, single or first primary malignancy, Results: The analytic cohort included 6,564 patients, with 4,315 in development and 2,249 in temporal validation. Median age was 64 years, 5,189 patients (79.1%) were male, and liver metastasis was present in 3,249 patients (49.5%). Six-month CSM occurred in 1,941 patients (45.0%) in development and 902 patients (40.1%) in temporal validation. The selected XGBoost model achieved an area under the curve (AUC) of 0.808 [95% confidence interval (CI), 0.795-0.821] in development and 0.786 (95% CI, 0.767-0.806) in temporal validation. In temporal validation, the Brier score was 0.173 versus a null Brier score of 0.240, the calibration slope was 1.130, and the integrated calibration index was 0.026. At the high-risk threshold, sensitivity was 56.7%, specificity was 90.2%, positive predictive value was 79.5%, and negative predictive value was 75.7%. Observed six-month CSM was 20.2%, 32.1%, and 79.5% in low-, intermediate-, and high-risk groups, respectively. Conclusions: A SEER-based prediction model was developed and temporally validated for six-month CSM in de novo metastatic GEJA. The model provided moderate discrimination, acceptable calibration, and clinically distinct population-level risk strata. Because validation was limited to a later SEER cohort, the model should be externally validated and locally recalibrated before clinical workflow use, and it must not be interpreted as evidence of treatment efficacy.