Anatomic Neuroimaging Characteristics of Posterior Fossa Type A Ependymoma Subgroups

Abstract

Background and purpose: Posterior fossa type A (PFA) ependymomas have 2 molecular subgroups (PFA-1 and PFA-2) and 9 subtypes. Gene expression profiling suggests that PFA-1 and PFA-2 tumors have distinct developmental origins at different rostrocaudal levels of the brainstem. We, therefore, tested the hypothesis that PFA-1 and PFA-2 ependymomas have different anatomic MR imaging characteristics at presentation.

Materials and methods: Two neuroradiologists reviewed the preoperative MR imaging examinations of 122 patients with PFA ependymomas and identified several anatomic characteristics, including extension through the fourth ventricular foramina and encasement of major arteries and tumor type (midfloor, roof, or lateral). Deoxyribonucleic acid methylation profiling assigned ependymomas to PFA-1 or PFA-2. Information on PFA subtype from an earlier study was also available for a subset of tumors. Associations between imaging variables and subgroup or subtype were evaluated.

Results: No anatomic imaging variable was significantly associated with the PFA subgroup, but 5 PFA-2c subtype ependymomas in the cohort had a more circumscribed appearance and showed less tendency to extend through the fourth ventricular foramina or encase blood vessels, compared with other PFA subtypes.

Conclusions: PFA-1 and PFA-2 ependymomas did not have different anatomic MR imaging characteristics, and these results do not support the hypothesis that they have distinct anatomic origins. PFA-2c ependymomas appear to have a more anatomically circumscribed MR imaging appearance than the other PFA subtypes; however, this needs to be confirmed in a larger study.

FIG 1.

PFA ependymoma types based on location. Sagittal T1- (A) and axial T2-weighted images (B): Midfloor-type posterior fossa ependymomas fill the obex (arrowhead), and a gap may be seen between the roof of the fourth ventricle and the tumor (arrows, A). The brainstem is displaced anteriorly (arrow, B), and in the sagittal plane, it is seen in its entirety in a single sagittal image. Sagittal T1- (C) and axial T2-weighted images (D): Lateral-type posterior fossa ependymomas are centered in the lateral recess of the fourth ventricle or cerebellopontine angle (arrows, C) and displace the brainstem laterally (arrow, D). The brainstem usually cannot be seen in a single sagittal image. Postgadolinium sagittal T1- (E) and axial T2-weighted images (F): Roof-type posterior fossa ependymomas appear closely associated with the roof of the fourth ventricle (arrows, E and F) and do not fill the obex (arrowhead, E). A gap is usually seen between the tumor and the floor of the fourth ventricle (arrowhead, F).

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.