Interpretation and generalizability
Compared to VBM, the ABC/2 formula overestimated volumes in 63% of cases. Similarly, in a study on the natural history of petroclival meningiomas, the volume estimated by the linear method was on average 1.6 times larger than that estimated using ROI and threshold-based VBM segmentation. The authors suggested that the irregular shape of meningiomas, which often differs from the shape of an ellipsoid, a characteristic necessary for accurate estimation by the ABC/2 formula, would be responsible for the overestimation [16].
Semiautomated segmentation is considered more reliable than other volumetric methods to assess tumor growth since meningiomas may present areas with higher growth rates than the rest of the tumor, resulting in an irregular shape [6]. Besides, the volumetric criterion of tumor progression, given by the volume calculated using semiautomated segmentation, strongly correlates with overall survival compared to 1D and 2D volumetric approaches, although the difference was modest [6].
Although error percentage had no association with linear volume estimation, smaller tumors exhibited greater error indices in planimetry. This is partly due to surrounding voxels included when manually contouring the tumor edges, making small lesions proportionally more affected by the inclusion of nearby tissue. This effect is corrected for in multiparametric segmentation. In T1CE images, the interface between the tumor and surrounding structures may become slightly broader to the naked eye due to signal interpolation on the DICOM viewer software. For this reason, signal intensity fades at the tumor-parenchyma interface, making it difficult to define the limits precisely. Multiparametric VBM is not susceptible to this pitfall for two main reasons: (1) segmentation is based on voxel numerical signal intensity, eliminating the susceptibility to visual artifacts, such as blurring, and (2) tissue segmentation is based on the voxel-by-voxel signal intensity from multiple acquisition sequences, removing the hypersignal bias from a T1CE-only border definition.
In our cohort, planimetry revealed larger volume than linear measurements in 70% of cases. Contrarily, a previous study reported a larger volume in ABC/2 compared to planimetry in 76% of cases [3]. Tumor shape heterogeneities may also explain such discrepancies between samples since ABC/2 does not account for tumor surface irregularities.
Linear measurement error correlated only with tumor roundness, regardless of tumor size. In linear estimations, roundness increased measurement accuracy because the method used to determine tumor volume is based on the assumption of tumor sphericity, intrinsic to the ABC/2 formula. Thus, the closer to a sphere or ellipsoid shape, the greater the estimation accuracy. In this regard, we have found that roundness needs to be greater than 0.6 for an adequate assessment using the linear method. However, deciphering the roundness in clinical practice may be a challenge. For this purpose, Krumbein’s chart for visual determination of roundness might be used to predetermine eligible tumors (Fig. 3) [17]. With regards to flatness, it was shown to reduce planimetric error. When manually drawing along the tumor edges, this effect may also be explained by the overinclusion of surrounding voxels. Considering tumor volume as the sum of each layer’s ROI, flat tumors end up including fewer surrounding voxels as fewer layers are drawn during segmentation.
From a clinical standpoint, meningioma growth rate is crucial for therapeutic decisions. Therefore, a practical and accurate volume measurement method is valuable for neurosurgeons and neuro-oncologists. Chang et al. [8] identified that the modified ellipsoid formula produced lower tumor growth rates, detecting significant tumor growth in 12 of 29 patients. In comparison, the planimetric method detected 19 patients with significant growth, meaning that planimetry is more sensitive than ABC/2 in detecting tumor growth for follow-up purposes. Notably, the planimetric method is much more time-consuming than the ABC/2 method and can exhibit considerable inter-rater variability due to the difficulty in distinguishing tumor margins with bone involvement and skull base tumors.[6, 18, 19] In addition to the volume measurement methods cited above, fully automated volumetric models for meningioma segmentation have recently been developed, with high accuracy and reliability [18, 19].
The best volumetric assessment for meningiomas is yet to be defined. Response assessment criteria for meningioma are still in progress by the Response Assessment in Neuro-oncology (RANO) committee [20]. However, considering previous RANO criteria for progressive high-grade gliomas [15] and brain metastases [14], which used bi- and unidimensional measurements, respectively, the role of our tridimensional findings in terms of the median absolute error obtained by manually-based methods (up to 14%) is highly relevant and should be included in the discussion. Even though its use in clinical practice is not yet widespread, VBM has become more accessible and more straightforward by using MRI straight from a hospital Picture Archiving and Communication System (PACS).
Planimetry is time-consuming, especially for the layer-by-layer contouring of large and irregular tumors. On the other hand, linear measurements can be done in minutes, but it ends up oversimplifying tumors’ sometimes complex and irregular shape. Furthermore, considering that meningiomas cause symptoms by compression of nearby neurovascular structures, and that larger tumors tend to be less rounded, it is essential to ponder the advantages and limitations of this method. VBM merges the benefits of a relatively fast segmentation, taking about 5–10 minutes by tumor, and a machine learning-aided accurate measurement. In this sense, considering the available options and choosing the most adequate method have become the two most important tasks when dealing with patients with meningiomas, especially when the growth rate needs to be closely monitored.