Background. The purpose of this study was to assess the reproducibility of the previously describedT2–fluid attenuated inversion recovery (FLAIR) mismatch sign as a specific imaging marker in non-enhancing isocitrate dehydrogenase (IDH) mutant, 1p/19q non-codeleted lower-grade glioma (LGG), encompassing both diffuse and anaplastic astrocytoma. Methods. MR scans (n = 154) from 3 separate databases with genotyped LGG were evaluated by 2 independent reviewers to assess (i) presence/absence of “T2-FLAIR mismatch” sign and (ii) presence/absence of homogeneous signal onT2-weighted images. Interrater agreement with Cohen's kappa (κ) was calculated, as well as diagnostic test performance of theT2-FLAIR mismatch sign to identify IDH-mutant astrocytoma. Results. There was substantial interrater agreement for theT2-FLAIR mismatch sign [κ = 0.75 (0.64–0.87)], but only fair agreement forT2 homogeneity [κ = 0.38 (0.25–0.52)].TheT2-FLAIR mismatch sign was present in 38 cases (25%) and had a positive predictive value of 100%, negative predictive value of 68%, a sensitivity of 51%, and a specificity of 100%. Conclusions. With a robust interrater agreement, our study confirms that among non-enhancing LGG theT2-FLAIR mismatch sign represents a highly specific imaging marker for IDH-mutant astrocytoma.This non-invasive marker may enable a more informed patient counsel and can aid in the treatment decision processes in a significant proportion of patients presenting with non-enhancing, LGG-like lesions.

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