Preoperative Prediction of High Mitotic Count in Gastrointestinal Stromal Tumors Using CT Features and Serologic Indicators: An Interpretable Model with Multicenter External Validation.

Background: Accurate preoperative identification of high mitotic count is important for risk stratification in gastrointestinal stromal tumors (GISTs), yet biopsy is invasive and conventional imaging alone does not directly reflect microscopic proliferative activity. We aimed to develop and externally validate an interpretable model integrating CT features and serologic indicators. Methods: This multicenter retrospective study included 802 patients from three Shanxi hospitals, randomly split into a training cohort (n = 562) and an internal validation cohort (n = 240), plus an external validation cohort from Dalian (n = 255). LASSO and multivariable logistic regression were used for feature selection and nomogram construction. Five individual machine-learning models and 31 stacking ensembles were trained, and SHAP was used to interpret model behavior. Results: Tumor size, liquefaction/necrosis, coarse vessel sign, peritumoral fat stranding, platelet-to-lymphocyte ratio, and albumin-to-fibrinogen ratio were independent predictors of high mitotic count. Among individual models, SVM achieved the highest internal-validation AUC (0.866). The best stacking model (SVM+ANN+Logit) reached an AUC of 0.867 on internal validation and 0.955 on external validation, with good calibration and favorable decision-curve performance. The gain over the best single model was small but consistent. Conclusion: An interpretable model combining CT and serologic features may provide a practical non-invasive tool for preoperative estimation of mitotic count in GISTs. Prospective validation is still needed before routine clinical implementation.