Preoperative Prediction of Residual Cancer Burden After Neoadjuvant Chemotherapy in Breast Cancer: A Multimodal Machine Learning Approach and Implications for Clinical Decision Support

Background. Residual cancer burden (RCB) after neoadjuvant chemotherapy (NAC) offers finer prognostic stratification than binary pathologic complete response, and increasingly guides adjuvant treatment intensity. Predicting four-tier RCB class from preoperative data could inform adjuvant planning before surgery, yet this remains an unmet need; and when two models reach equal discrimination, the key question is which generalizes most reliably. We compared a radiology-focused model with a fully integrated multimodal model for preoperative four-class RCB prediction. Methods. In a single-center, retrospective cohort of 328 patients treated with NAC followed by surgery, 64 clinicopathologic and radiologic variables were organized into thematic blocks. Two configurations were compared: a 17-variable radiology model (Model R) and a 62-variable multimodal model (Model ALL). Three algorithms (Random Forest, XGBoost, LightGBM) were evaluated with and without SMOTE using an 80/20 stratified split and 5-fold cross-validation. Model selection combined test AUC, macro-F1, cross-validation-to-test gap, nested cross-validation, bootstrap confidence intervals, and SHAP explainability, following the TRIPOD+AI guidance. Results. RCB classes were distributed as RCB-0 27.4% (n=90), RCB-I 10.4% (n=34), RCB-II 43.6% (n=143), and RCB-III 18.6% (n=61). Model R and Model ALL reached identical test AUC (0.838). Model ALL, however, achieved higher accuracy (0.636 vs 0.530) and macro-F1 (0.602 vs 0.598), together with a substantially smaller cross-validation-to-test gap (0.015 vs 0.099), pointing to more stable generalization; this gap difference persisted across all three algorithms. SHAP analysis showed that the multimodal model drew jointly on imaging phenotype, tumor biology, and disease extent. Both models remained weakest in the RCB-III class. Conclusions. At equivalent discrimination, the multimodal model was methodologically preferable for preoperative RCB prediction, owing to its stability and interpretability - qualities relevant to trustworthy clinical decision support. It remains investigational; a model flagging likely RCB-0 or RCB-III before surgery could prioritize adjuvant-therapy discussions earlier in the care pathway, pending prospective external validation.