We propose a Foundation-based Biomarker Network (FoundBioNet) for accurate and noninvasive detection of IDH mutations. This model, based on the SWIN-UNETR architecture, integrates the Tumor-Aware Feature Encoding (TAFE) module for tumor-centric feature extraction and the Cross-Modality Differential (CMD) module, which highlights subtle T2-FLAIR mismatch signals associated with IDH mutations, to noninvasively predict IDH mutation status from multiparametric MRI. Using a 1,705-patient dataset, our results demonstrate high AUC values across diverse datasets, outperforming existing methods. We demonstrate the importance of the TAFE and CMD modules, and our model enables generalizable glioblastoma characterization through large-scale pretraining and task-specific fine-tuning.