Comparing size evaluation methods for acoustic neuroma after stereotactic radiosurgery

Assessing Tumor Volume for Sporadic Vestibular Schwannomas: A Comparison of Methods of Volumetry

INTRODUCTION

The size of vestibular schwannomas (VS) is a major factor guiding the initial decision of treatment and the definition of tumor control or failure. Accurate measurement and standardized definition are mandatory; yet no standard exist. Various approximation methods using linear measures or segmental volumetry have been reported. We reviewed different methods of volumetry and evaluated their correlation and agreement using our own historical cohort.

METHODS

We selected patients treated for sporadic VS by Gammaknife radiosurgery (GKRS) in our department. Using the stereotactic 3D T1 enhancing MRI on the day of GKRS, 4 methods of volumetry using linear measurements (5-axis, 3-axis, 3-axis-averaged, and 1-axis) and segmental volumetry were compared to each other. The degree of correlation was evaluated using an intraclass correlation test (ICC 3,1). The agreement between the different methods was evaluated using Bland-Altman diagrams.

RESULTS

A total of 2,188 patients were included. We observed an excellent ICC between 5-axis volumetry (0.98), 3-axis volumetry (0.96), and 3-axis-averaged volumetry (0.96) and segmental volumetry, respectively, irrespective of the Koos grade or Ohata classification. The ICC for 1-axis volumetry was lower (0.72) and varied depending on the Koos and Ohata subgroups. None of these methods were substitutable.

CONCLUSION

Although segmental volumetry is deemed the most accurate method, it takes more effort and requires sophisticated computation systems compared to methods of volumetry using linear measurements. 5-axis volumetry affords the best adequacy with segmental volumetry among all methods under assessment, irrespective of the shape of the tumor. 1-axis volumetry should not be used.

Follow-up assessment of vestibular schwannomas: volume quantification versus two-dimensional measurements

Introduction

A conservative treatment strategy is often proposed as a primary treatment option in the management of vestibular schwannomas (VS). In this “wait and scan” policy, audiovestibular symptoms are monitored regularly, and VS growth is measured on consecutive magnetic resonance images (MRI). The aim of this study is validation of two-dimensional versus volume MRI assessment in the longitudinal follow-up of VS and to define tumor growth beyond measurement error.

Methods

MRI scans of 68 consecutive patients with VS were analyzed retrospectively. Two-dimensional and volume measurements on contrast enhanced (CE) T1- and T2-weighted images were performed independently by two readers. Smallest detectable differences (SDD) were calculated, and intraclass correlation coefficients (ICCs) were determined for both assessment methods.

Results

Two-dimensional and volume measurements both showed best reproducibility on CE T1-weighted images. SDD for differences relative to baseline MRI [SDD (%)] for two-dimensional measurements had a higher interobserver error compared to volume measurements (40% versus 19.7%), which decreases when tumor size increases. The ICC for two-dimensional measurements in three directions was 0.947, 0.974, and 0.978 and for volume measurements 0.999.

Conclusion

Volume measurements are more accurate compared to two-dimensional measurements for the evaluation of VS growth. These measurements are assessed preferably on CE T1-weighted images. SDD (%) strongly depends on VS size. SDD between consecutive scans exceeds the common clinical applied criterion of 1 or 2 mm growth to define growth.