Arterial Spin Labeling-Derived Fractional Tumor Burden as a Primary Biomarker for Differentiating Recurrent Tumor from Radiation Necrosis After Stereotactic Radiosurgery

BACKGROUND AND PURPOSE: Differentiating recurrent tumor from radiation necrosis after stereotactic radiosurgery remains a challenge with conventional MRI. Fractional tumor burden, a voxel-level perfusion biomarker originally developed using DSC-MRI, characterizes intralesional perfusion heterogeneity and has demonstrated good performance and improved interrater agreement in diagnostic interpretations over conventional perfusion metrics. We evaluated arterial spin labeling-derived fractional tumor burden for distinguishing tumor recurrence from radiation necrosis in treated brain metastases. MATERIALS AND METHODS: This retrospective study included 86 patients with 102 brain metastases (68 radiation necrosis, 34 tumor) evaluated with arterial spin labeling-MRI after stereotactic radiosurgery. Ground truth was based on histopathology or clinico-radiologic follow-up. The primary endpoint was fractional tumor burden-high voxel percentage (absolute and normalized). Secondary endpoints included fractional tumor burden-low, mean CBF, maximum CBF, and delta T1. Absolute CBF thresholds of ≤50 and ≥80 mL/100 g/min and normalized thresholds of ≤1.0 and ≥1.4 were defined a priori from prior histopathology-informed data. Diagnostic performance was assessed by receiver operating characteristic analysis and DeLong testing. Between-group comparisons used Mann-Whitney U tests; generalized linear mixed-effects models assessed logistic regression. CONCLUSIONS: Arterial spin labeling-derived fractional tumor burden-high provides good lesion-level differentiation between recurrent brain metastases and radiation necrosis after stereotactic radiosurgery, performing comparably to conventional CBF metrics. The voxel-level approach offers a clinically interpretable perfusion biomarker suitable for post-treatment response assessment.