Imaging MRI Biomarkers for Diagnosing Brain Radionecrosis: Focus on Incomplete Ring Enhancement Sign

PURPOSE Differentiating brain radionecrosis from recurrent metastases after radiotherapy remains challenging due to the lack of definitive imaging tool modality. This study evaluated the diagnostic accuracy of the “incomplete ring enhancement sign” on contrast-enhanced 3D T1-weighted gradient-echo sequence (CE 3D T1 GRE) as a biomarker of radionecrosis and analyzed additional associated features. METHODS In this retrospective case-control study, 45 cases of radionecrosis were identified from an institutional database of 1,179 brain metastases consecutively treated with stereotactic radiotherapy (2015–2022). Ninety CE 3D T1 GRE (45 radionecrosis and 45 matched metastases controls from the same patients obtained before radiotherapy) were randomly reviewed under blinded conditions, as observers were unaware of whether each case corresponded to metastases or to radionecrosis. The presence of an incomplete ring enhancement sign was assessed, and diagnostic performance was calculated. Interobserver agreement was evaluated using Fleiss’s kappa. Fragmentation, satellite lesions, and lesion distribution were compared using χ². RESULTS Sensitivity was 97% (95% CI, 87–99) and specificity 76% (95% CI, 61–88). Interobserver agreement was good (k = 0.75). Incomplete ring enhancement appeared in 50% of the MRIs where radionecrosis was initially suspected. Fragmentation occurred in 89% of radionecrosis cases compared to 2% of metastases ( p  < 0.001), and satellite lesions in 29% vs. 2%, respectively ( p  < 0.001). Watershed localization was found in 67% of radionecrosis cases. CONCLUSION The incomplete ring enhancement sign is an early and reliable imaging biomarker of brain radionecrosis supported by associated features such as fragmentation, satellite foci, and watershed localization.