Percentage signal recovery plus relative cerebral blood volume: a practical dual-parameter strategy for differentiating post-stereotactic radiosurgery tumour progression from radiation necrosis in brain metastases

OBJECTIVES: Stereotactic radiosurgery (SRS) is widely used for brain metastases, but differentiating tumour progression from radiation necrosis on conventional MRI remains difficult. Percentage signal recovery (PSR), derived from dynamic susceptibility contrast (DSC) perfusion MRI, reflects signal recovery post-contrast and offers insights into capillary permeability. This study aimed to evaluate PSR and relative cerebral blood volume (rCBV) and assess their combined diagnostic value in post-SRS brain metastases. MATERIALS AND METHODS: Patients with enlarging post-SRS brain metastases and diagnostic uncertainty were retrospectively included. PSR and rCBV were extracted from DSC-MRI and normalised to contralateral white matter. The dataset was split into training and validation cohorts using stratified sampling. Logistic regression with 5-fold cross-validation and bootstrap validation was used. Diagnostic performance was assessed by ROC analysis. RESULTS: Sixty-one patients (62 lesions; 26 progression, 36 necrosis) were included. Inter-rater reliability was excellent (ICC > 0.90). Progression showed higher rCBV (2.84 vs. 0.76) and lower PSR (95% vs. 176%) (both p < 0.001). Both were significant in univariate analysis; PSR remained independently predictive (p = 0.04) in multivariate analysis. PSR outperformed rCBV and the combined model in ROC analysis (validation AUCs: 0.960, 0.898, and 0.945, respectively), while the combined PSR and rCBV model maintained excellent sensitivity, specificity, and overall accuracy. Bootstrap-derived thresholds were 108% (PSR) and 1.96 (rCBV). A nomogram was developed for individualised risk estimation. CONCLUSIONS: PSR and rCBV provide complementary diagnostic information for post-SRS lesion assessment. PSR may offer additional value without requiring extra image acquisition, and integration of both parameters could enhance diagnostic confidence. Routine inclusion of PSR and rCBV in post-SRS imaging protocols could be recommended. CRITICAL RELEVANCE STATEMENT: This study demonstrates that combining DSC MRI-derived rCBV and PSR improves accuracy and efficiency in distinguishing tumour recurrence from radiation necrosis, offering a practical dual-parameter approach to enhance diagnosis and guide timely clinical decision-making in neurooncology. KEY POINTS: DSC MRI-derived rCBV and PSR may aid in improving diagnostic accuracy and reducing time-to-diagnosis in post-SRS brain metastases. PSR can be derived from the same DSC acquisition without additional scanning or correction and represents a practical parameter for post-SRS lesion assessment. Combining rCBV and PSR may improve diagnostic confidence, especially in equivocal cases, supporting routine use of this dual-parameter model.