Regarding "The “Outline Sign”: Thin Hyperenhancing Perimeter as an MR Imaging Feature of Meningioma. A Useful Tool in the Temporal Bone Region for Differentiating Meningiomas from Schwannomas and Paragangliomas"

We read with great interest the article by Vasireddi et al., "The “Outline Sign”: Thin Hyperenhancing Perimeter as an MR Imaging Feature of Meningioma. A Useful Tool in the Temporal Bone Region for Differentiating Meningiomas from Schwannomas and Paragangliomas", which investigates the utility of the "Outline Sign" for distinguishing meningiomas from schwannomas and paragangliomas in the temporal bone region. Indeed, this is a valuable effort addressing a relevant and frequent diagnostic challenge in neuroradiology. With the aim of boosting the value of this interesting article even more, we would like to raise some methodological aspects and suggest additional considerations regarding the final conclusions.

First, we stress the potential impact of variability in the timing between contrast administration and image acquisition, which the authors did not account for in their methodology, something that could potentially and significantly influence the depiction of the "Outline Sign." Delayed imaging after contrast administration, for instance, may exaggerate or diminish peripheral enhancement due to differences in contrast perfusion and diffusion dynamics.[1,2] This raises an important question: could the observed "Outline Sign" be influenced, at least in part, by inconsistencies in the imaging protocols rather than being an intrinsic tumour characteristic? Addressing this issue through standardized imaging protocols or subgroup analyses could improve the reliability of this finding.

Second, while the authors acknowledge that tumours from various anatomical locations were included, we believe this heterogeneity potentially biases the hypothesis that the "Outline Sign" is a reliable diagnostic tool for the temporal bone region. Differences in vascularization, tumour morphology, and surrounding anatomical structures between locations may confound the results.[3] Consequently, the study does not unequivocally demonstrate the effectiveness of the sign as a differential diagnostic tool specifically for tumours in the temporal bone region; this conceptual misalignment between the main objective and the study data is also supported by the inclusion of a meningioma example from a non-temporal location.

Finally, the small sample size of temporal bone tumours, particularly schwannomas and paragangliomas, limits the statistical power and generalizability of the findings. A larger, more targeted cohort restricted to tumours in the temporal bone region would provide stronger evidence and enhance the validity of the conclusions. We commend the authors for their effort in exploring a potentially valuable diagnostic feature. Addressing the aspects outlined above in future studies would further clarify the utility of the "Outline Sign" and its role in clinical practice.

References:

1. Seo J, Lim C, Lee KY, Koh YC, Moon WJ. Time optimization of gadobutrolenhanced brain MRI for metastases and primary tumors using a dynamic contrastenhanced imaging. BMC Med Imaging. 2022;22(1):180. https://doi.org/10.1186/S12880-022-00909-Z

2. Piechotta PL, Bonekamp D, Sill M, et al. Increased Delay Between Gadolinium Chelate Administration and T1-Weighted Magnetic Resonance Imaging Acquisition Increases Contrast-Enhancing Tumor Volumes and T1 Intensities in Brain Tumor Patients. Invest Radiol. https://doi.org/10.1097/RLI.0000000000000432

3. Hamilton BE, Salzman KL, Patel N, et al. Imaging and Clinical Characteristics of Temporal Bone Meningioma. AJNR Am J Neuroradiol. 2006;27(10):2204. Accessed January 11, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC7977196/