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PURPOSE:Nasopharyngeal inflammatory pseudotumor (NIPT) is hard to differentiate from infiltrating nasopharyngeal carcinoma (NPC) on conventional magnetic resonance imaging (MRI). The purpose of this study is to determine whether discrepant lesion sizes estimated on T1- and fat-suppressed T2-weighted images can help distinguish between NIPT and NPC.METHODS:We retrospectively reviewed MRI data of histologically proven 14 NIPTs and 18 infiltrating NPCs. We measured the area of the lesion on contrast-enhanced T1-weighted, unenhanced T1-weighted, and fat-suppressed T2-weighted images by placing the largest possible polygonal region-of-interest within the lesion at the same level. Using lesion size measured on contrast-enhanced T1-weighted image as the reference, we calculated and compared area ratio of T1 (ART1) and area ratio of T2 (ART2) between NIPTs and NPCs. For validation, we also undertook a double-blinded study by two reviewers and assessed the diagnostic performance and interobserver agreement.RESULTS:For NIPTs, ART2 (median, 0.48; range, 0.18–0.97) was statistically significantly less than ART1 (median, 1.01; range, 0.80–1.99), while these values were not significantly different for NPCs. The interobserver agreement in differentiating between NIPT and NPC was good, with a sensitivity of 93% and a specificity of 83%–94%.CONCLUSION:In contrast to NPCs, NIPTs appear smaller on fat-suppressed T2-weighted images than on T1-weighted images. This discrepancy in the lesion size estimated on T1-weighted and fat-suppressed T2-weighted images may provide a simple and consistent way to differentiate between NIPTs and NPCs on conventional MRI.