MR Imaging Grading System for Skull Base Chordoma

Discussion

In this retrospective study, the MR SI of patients with SBC was incorporated into statistical analyses, and SBCs were classified into 3 MR grades according to R_T2 and R_EN values. It was found that MR grade III tumors had more abundant blood supply than MR grade I tumors. Furthermore, the risk of SBC progression increased by 2.071 times for every single MR grade increase in the MR classification system.

MR imaging is useful not only for diagnosing disease but also in predicting prognosis and tumor classification. The MR SI and SI ratio of a lesion have been considered important indicators in many studies.^{18⇓⇓–21} However, chordomas show substantial heterogeneity on MR images, and MR imaging has previously been used only to aid in the diagnosis of SBC. No previous studies have reported the relationships between MR imaging features and SBC outcomes.

In the present study, tumor and pons signal intensities were measured in the PACS with a straightforward measurement process that ensures the acquisition of accurate data.²² We calculated R_{T1 FLAIR}, R_T2, and R_EN values to represent the tumor SI in each sequence while eliminating confounding factors caused by MR imaging conditions among different patients. Although there was no evidence that the MR signal ratio is a risk factor for mortality in statistical analysis, our results showed that a higher R_T2 value predicted diminished tumor progression and that a higher R_EN value predicted more rapid tumor progression. We therefore established MR grades on the basis of these values. Multivariate analysis of tumor progression indicated that MR grade was a significant risk factor and that the risk of SBC progression increased as the MR grade increased.

Chordomas can be classified into conventional, chondroid, and dedifferentiated types based on histopathologic manifestations. Conventional chordoma is the most common type, and chondroid and dedifferentiated chordomas exhibit the most favorable and worst outcomes, respectively.^9,23,24 Our results are consistent with those in previous reports. Although pathologic confirmation is the criterion standard for the diagnosis of SBC and pathologic type is an independent variable in the PFS analysis, the MR grading method for SBC was more useful for predicting the prognosis of patients with SBC. This usefulness derived from the following: First, there was a relatively uniform distribution of the 3 MR grades of SBC; second, the MR grade can be immediately determined after an initial MR imaging examination, which is far earlier than a pathologic diagnosis; and third, the grading process is simple because the measurement of SI and the following calculation are both straightforward. This feature makes this technique more accessible to neurosurgeons and radiologists.

Prior studies have discussed the association between MR SI and lesion blood supply.²⁵ They found that tumors with an abundant blood supply have lower R_T2 values. We produced similar results in the present study, showing that MR grade III tumors (low R_T2 and high R_EN) had a more abundant blood supply than MR grade I tumors (high R_T2 and low R_EN) and that the former are associated with greater intraoperative blood loss than the latter. This finding is useful for preoperative evaluations of tumor blood supply and is of great significance for clinical practice. Prior knowledge of this factor may alter therapy, such as in preparing adequate quantities of blood, altering a planned operative approach, and in anticipating other measures for potentially significant hemorrhage.

In the present study, chondroid SBCs were associated with a longer PFS than conventional SBCs, and this finding is consistent with those in prior studies. However, SBC pathologic type is not significantly associated with MR grade. Indeed, MR SI did not differ between chondroid and conventional SBCs, and no other studies have reported differences in MR imaging features between different SBC pathologic types. This finding may be because differences in histopathologic levels are not necessarily reflected in imaging modalities. These results need to be confirmed in additional studies.

We enrolled 156 patients with primary SBC in this study, and to the best of our knowledge, this represents the largest primary SBC single-center study. The single-center source avoided the heterogeneity occasionally encountered in multicenter studies. Additionally, we demonstrated the utility of MR SI ratios for SBC grading and confirmed the validity of our MR grading system. We also have demonstrated that the simple ratio of the tumor to the pons allows easy determination of SBC MR grade. This grading system can be widely used. Additionally, the system was useful for predicting the progression of SBC in early tumor stages. Finally, these ratios can also be used to evaluate tumor blood supply, which is of therapeutic significance.

There were limitations to the current study. First, the relatively short follow-up was not sufficient for analyzing long-term outcomes; accordingly, the incidence of terminal events was relatively low for the OS and PFS. Second, the patients were grouped on the basis of retrospective assessments; therefore, further prospective studies are needed to validate the effectiveness of the grading system.