Imaging MRI biomarkers for diagnosing brain radionecrosis: focus on incomplete ring enhancement sign

Differentiating brain radionecrosis from recurrent metastases after radiotherapy remains challenging because no single imaging technique is definitive. This study evaluated the diagnostic accuracy of the incomplete ring enhancement sign (IRES) on contrast-enhanced 3D T1-weighted gradient-echo MRI (CE 3D T1 GRE) as a diagnostic sign of radionecrosis and analyzed associated morphologic features.

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 examinations were reviewed in blinded fashion: 45 radionecrosis examinations and 45 matched pre-treatment-naïve metastases from the same patients. The presence of IRES 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.

Mean sensitivity and specificity against treatment-naïve metastases were 97% (95% CI, 87-99) and 76% (95% CI, 61-88), respectively. Interobserver agreement was substantial (κ = 0.75). IRES was already present in 50% of the MRIs in which radionecrosis was first suspected. Fragmentation occurred in 89% of radionecrosis cases versus 2% of metastases (p < 0.001), and satellite lesions in 29% versus 2% (p < 0.001). Watershed localization was found in 83% of supratentorial radionecrosis. Because the control group consisted of treatment-naïve metastases rather than post-radiotherapy recurrences, the reported accuracy should be interpreted as proof-of-concept.

The incomplete ring enhancement sign is a potential diagnostic sign of brain radionecrosis. Fragmentation, satellite foci, and watershed localization may reinforce the diagnosis.