Individualized Per-Site Meta-Federated Feature Learning (iPS-MFFL) for Privacy-Preserving Brain Tumor MRI Classification under non-IID Heterogeneity

BackgroundFederated learning (FL) enables collaborative model training across institutions without sharing patient-level data. However, standard FL algorithms such as FedAvg degrade under non-independently and non-identically distributed (non-IID) data, a prevalent condition when patient demographics, scanner hardware, and disease prevalence differ across hospital sites. ObjectiveWe propose iPS-MFFL (Individualized Per-Site Meta-Federated Feature Learning), a federated framework with a hierarchical local-model architecture that addresses non-IID heterogeneity through (1) a shared feature extractor, (2) multiple weak-learner classification heads that can be trained with heterogeneous training objectives to promote complementary decision boundaries, (3) independent per-learner server aggregation so that each weak learners parameters are averaged only with its counterparts at other clients, and (4) a lightweight meta-model -- itself federated -- that adaptively stacks the weak-learner outputs. The specific choices of backbone, weak-learner training objectives, and meta-model are implementation details; in this work we use an ImageNet-pretrained ResNet18 and three heterogeneous losses as a concrete instantiation. MethodsWe evaluate on the Brain Tumor MRI Classification dataset (7,200 images; 4 classes: glioma, meningioma, pituitary tumor, no tumor) partitioned across K = 5 simulated hospital sites using Dirichlet non-IID sampling ( = 0.3). Four baselines are compared: Local-only training, FedAvg, FedProx, and Freeze-FT. All experiments are repeated over three random seeds (13, 42, 2025) and evaluated using paired t-tests, Cohens d effect sizes, and post-hoc power analysis. ResultsiPS-MFFL achieved the highest mean final-round test accuracy point estimate of 85.42 {+/-} 8.70% (mean {+/-} SD across three seeds), compared to FedAvg (78.48 {+/-} 12.66%), FedProx (78.33 {+/-} 14.64%), Freeze-FT (73.98 {+/-} 21.09%), and Local (58.10 {+/-} 11.77%). iPS-MFFL showed the smallest cross-seed SD, suggesting greater robustness to partition heterogeneity. However, one-way ANOVA did not reach statistical significance (F = 1.52, p = 0.270), reflecting the limited number of experimental seeds. Cohens d effect sizes relative to iPS-MFFL ranged from 0.59 (vs. FedProx) to 2.64 (vs. Local); post-hoc pairwise comparisons are reported as exploratory given the non-significant omnibus test. Post-hoc power analysis indicated that statistical power for FL baseline comparisons was only 0.10-0.12 for the observed effect sizes (d {approx} 0.6) at n = 3 seeds. ConclusionsiPS-MFFL provides a practical approach to heterogeneous federated brain tumor classification by combining transfer learning, contrastive weak-learner diversity, and meta-learning. The framework demonstrated the highest mean accuracy and lowest variance across diverse data partitions. Validation with larger seed pools ([≥] 10 seeds for 80% power), ablation studies, and external multi-center cohorts is needed to establish generality.