Conventional MRI/CT for differentiation of IDH-mutant adult-type diffuse gliomas: revisited with a new imaging feature “scFLAIR-D”

Purpose To test whether a refined definition of the T2–FLAIR mismatch sign (redefined T2FM) and a newly defined subcortical FLAIR signal drop (scFLAIR-D) on conventional MRI improve presurgical discrimination between astrocytoma, IDH-mutant (AST) and oligodendroglioma, IDH-mutant and 1p/19q-codeleted (ODG). Materials and methods In this single-center retrospective study (January 2021–July 2025), adults with IDH-mutant adult-type diffuse glioma and preoperative T2-weighted and FLAIR MRI were included. Two blinded readers (with senior adjudication) scored five features: redefined T2FM (includes partial but excludes subcortical-only mismatch), scFLAIR-D (focal FLAIR hypointensity confined to subcortical white matter within T2-hyperintensity), multiple necrotic/cystic foci (≥ 2 cavities), cortical predominance (> 50% cortical volume), and calcification (CT). Results Forty-one patients were included (median age 42 years; 24 men; 16 AST, 25 ODG). AST significantly more often showed redefined T2FM (AST: 87.5% [14/16] vs. ODG: 20.0% [5/25]; p < 0.001) than ODG. ODG significantly more often showed scFLAIR-D (AST: 6.3% [1/16] vs. ODG: 44.0% [11/25]; p = 0.013), multiple necrotic/cystic foci (12.5% [2/16] vs. 56.0% [14/25]; p = 0.008), and calcification (0% [0/16] vs. 50.0% [12/24 patients with preoperative CT]; p < 0.001); cortical predominance trended higher (31.3% [5/16] vs. 64.0% [16/25]; p = 0.058). For predicting AST, redefined T2FM achieved 87.5% sensitivity and 80.0% specificity. For predicting ODG, scFLAIR-D achieved 44.0% sensitivity and 93.8% specificity. Two or more ODG-favored features (scFLAIR-D, multiple necrotic/cystic foci, calcification) identified ODG with 100% specificity, whereas redefined T2FM without ODG-favored features identified AST with 96.0% specificity. Conclusion A refined T2FM definition improves sensitivity for AST while preserving high specificity. scFLAIR-D serves as a simple, specific marker of ODG; in combination, these routine features yield high rule-in specificity to support presurgical planning and early therapeutic counseling. Secondary abstract Refining T2FM to include partial but exclude subcortical-only mismatch increased sensitivity for astrocytoma, IDH-mutant (87.5%) while preserving specificity (80.0%). Newly defined scFLAIR-D was highly specific for oligodendroglioma, IDH-mutant and 1p/19q-codeleted (93.8%). Combining these qualitative MRI/CT features yielded high rule-in specificities for lineage discrimination.