Discrepant lesion size estimated on T1- and fat-suppressed T2-weighted MRI: diagnostic value for differentiation between inflammatory pseudotumor and carcinoma of the nasopharynx

Abstract

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.

Figure 1. a–c

A representative case of nasopharyngeal inflammatory pseudotumor (NIPT). Axial contrast-enhanced (a) and unenhanced (b) T1-weighted images show an ill-defined, heterogeneously well-enhancing mass in the right infratemporal fossa. Axial fat-suppressed T2-weighted image (c) shows heterogeneous signal intensity of the mass. The low signal intensity in the posterior part of the lesion (c, asterisk), which is similar to the adjacent prevertebral muscle, results in underestimation of the size of the lesion. The AR_T1 and AR_T2 in this case were 0.90 and 0.66, respectively.

Figure 2. a–c

A representative case of nasopharyngeal carcinoma (NPC). Axial contrast-enhanced (a) and unenhanced (b) T1-weighted images show an ill-defined, heterogeneously enhancing mass in the nasopharynx and infratemporal fossa on the left. Axial fat-suppressed T2-weighted image (c) shows heterogeneous signal intensity of the mass, which is hyperintense to the adjacent muscle. The AR_T1 and AR_T2 in this case were 0.90 and 0.95, respectively.

Figure 3. a–c

A false negative case. Axial contrast-enhanced (a) and unenhanced (b) T1-weighted images show an ill-defined, heterogeneously enhancing mass in the left infratemporal fossa. Axial fat-suppressed T2-weighted image (c) shows the mass principally composed of hypointensity, comparable to the adjacent muscle. However, the peripheral rind of curvilinear hyperintensity surrounding the mass (arrows), which probably represents perilesional edema, leads us to draw the region-of-interest along it, resulting in no significant difference in the area measurements between A_CET1 and A_T2. The AR_T1 and AR_T2 in this case were 1.02 and 0.97, respectively. Both reviewers scored 3 for this lesion in favor of NPC. The lesion proved to be NIPT on histologic examination.

Figure 4. a–c

A false positive case. Axial contrast-enhanced (a) and unenhanced (b) T1-weighted images show an ill-defined, heterogeneously enhancing mass in the nasopharynx, skull base, and infratemporal fossa on the right. Axial fat-suppressed T2-weighted image (c) shows apparent hyperintensity of the mass in the infratemporal fossa. However, the signal intensity of the lesion in the clivus (asterisk) is indistinguishable from the unaffected portion of the clivus, resulting in underestimation of the size of the lesion on this fat-suppressed T2-weighted image. The AR_T1 and AR_T2 in this case were 0.91 and 0.67, respectively. The two reviewers scored 1 and 2 for this lesion, respectively, both in favor of NIPT. The lesion proved to be NPC on histologic examination.

Figure 5. a, b

Scatterplots displaying the various values of AR_T1 (a) and AR_T2 (b) in NIPTs and NPCs. While there is no reliable cutoff value of AR_T1 for differentiation between the two groups, with AR_T2 cutoff set at 0.69, NIPTs can be differentiated from NPCs with 86% sensitivity, 94% specificity, 92% PPV, 89% NPV, and 91% accuracy.