Anatomic Neuroimaging Characteristics of Posterior Fossa Type A Ependymoma Subgroups

DISCUSSION

Previous investigations of PF ependymomas have revealed associations between tumor location and operative or disease outcomes.^{4,5,17,18,22,23} For example, Ikezaki et al,¹⁷ U-King-Im et al,¹⁸ and Nagib and O’Fallon²² noted lower survival rates and greater difficulty in obtaining a gross total resection for laterally located tumors (those involving the lateral recess of the fourth ventricle and cerebellopontine angle). Sanford et al⁵ pointed out that cerebellopontine angle ependymomas in children arising from the foramen of Luschka and the lateral aspect of the medulla tend to encase cranial nerves and blood vessels and are difficult to resect completely, leading to lower survival rates. Sabin et al⁴ determined that pediatric PF ependymomas could be divided into 2 distinct anatomic groups based on the location of their centroids and that the groups were associated with differences in progression-free survival and central-versus-lateral tumor location. The results of these studies with respect to clinical outcomes of posterior fossa ependymomas contributed to our hypothesis that the anatomic characteristics of a pediatric PFA ependymoma might also help predict its subgroup. In fact, though no survival difference was shown among the subgroups, an imaging and operative analysis of 40 tumors described in the article by Pajtler et al⁹ outlining the PFA ependymoma subgroups and subtypes found that the PFA-1 lesions were more likely to be located laterally than the PFA-2 subgroup and that the origin of PFA-2 tumors was more frequently from the roof of the fourth ventricle than the origin of PFA-1 ependymomas, suggesting that the subgroups arise from cells at different brainstem levels. These prior investigations suggested that an imaging evaluation of a larger group of PFA ependymomas would result in identification of distinct anatomic markers for the PFA subgroups.

In this investigation, however, traditional anatomic characteristics seen on imaging could not be used to separate PFA tumors into their subgroups, and the findings do not provide support for the hypothesis that the PFA-1 and PFA-2 subgroups arise in different locations. The association of necrosis with subgroups is of uncertain significance. If different sites of origin for the PFA subgroups lead to different vascular supplies for the PFA-1 and PFA-2 tumors, this difference could, theoretically, cause the PFA-1 subgroup to have more tenuous neovascularization with a greater tendency to thrombosis and necrosis. Our results, however, suggest that gross anatomic characteristics of PFA ependymomas visible on MR imaging at presentation are not indicative of where PFA-1 and PFA-2 tumors arise. This outcome may reflect loss of visibility on MR imaging of key anatomic distinctions related to the PFA subgroups by the time a tumor has grown large enough to cause symptoms. Given their ability to capture more subtle differences between imaging characteristics of tumors, machine learning techniques and radiomics may be beneficial in future efforts to use imaging to classify PFA ependymomas by subgroup and determine their anatomic origins. These imaging approaches have recently been used with MR imaging to predict the subgroups of medulloblastomas.^24,25

Examination of the imaging characteristics of tumors among the PFA subtypes, while limited by the small number of subjects in some of the subtypes, revealed that a larger proportion of PFA-2c ependymomas did not involve fourth ventricle foramina and/or did not demonstrate vascular encasement compared with other subtypes. A greater proportion of PFA-2c tumors also showed a group of imaging characteristics contained in a flag variable than the other PFA subtypes. These variables (no extension below the foramen magnum, no encasement of vessels, and midfloor-type) suggest that the PFA-2c lesions tend to be more anatomically circumscribed than PFA ependymomas of other subtypes. PFA-2c tumors had the best survival profile of all the PFA subtypes in our earlier study,⁹ and this could be related to a greater proportion of that subtype having imaging characteristics, contained in the flag variable, associated with greater resectability and better outcomes.^{4,5,18,19,22,23} Although additional study of larger numbers of PFA-2c subtype ependymomas is needed before any firm conclusions can be made, the subtype appears to have a distinct appearance that may be associated with its better outcome compared with the other PFA subtypes (Fig 2). If accurate imaging prediction of the PFA-2c subtype can be achieved, it would provide a noninvasive method to aid in clinical decision-making and determining prognosis.

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FIG 2.

PFA-2c ependymoma versus another subtype. Postgadolinium sagittal T1- (A) and axial T2-weighted (C) MR images of a PFA-2c subtype ependymoma and sagittal T1- (B) and axial T2-weighted (D) MR images of another PFA ependymoma subtype (1b) show that the PFA-2c ependymoma has a more circumscribed appearance in that it does not wrap around the brainstem or extend below the foramen magnum in distinction from the PFA-1b subtype (arrow, B). The PFA-2c ependymoma in this case did not extend through the foramina of Luschka, unlike the PFA-1b tumor (arrows, D), or encase the basilar or vertebral arteries. This more circumscribed appearance was seen in the 5 PFA-2c tumors in our study, though evaluation of a larger number of this subtype is needed to confirm this appearance as a characteristic of PFA-2c ependymomas.

Limitations of this investigation include the heterogeneity of the preoperative MR imaging examinations, which were performed at different institutions and with inconsistent imaging techniques. This might have limited consistent evaluation of the tumors, including determination of anatomic areas of involvement. The studies were only included, however, if they provided adequate visualization of the posterior fossa structures of interest, and all determinations of tumor characteristics were made by 2 neuroradiologists, with any differences resolved by consensus. Another limitation was the difficulty in distinguishing midfloor-type tumors from roof-type tumors because both types are centered in the fourth ventricle and it was sometimes difficult to determine whether a gap between the tumor and roof or floor of the fourth ventricle was present. Thus, some tumors may have been misclassified as midfloor or roof type. This limitation may have been lessened by the use of consensus between the neuroradiologists for difficult cases.