MRI Characteristics of Ependymoblastoma: Results from 22 Centrally Reviewed Cases

BACKGROUND AND PURPOSE:Ependymoblastoma is a malignant embryonal tumor that develops in early childhood and has a dismal prognosis. Categorized by the World Health Organization as a subgroup of CNS-primitive neuroectodermal tumor, ependymoblastoma is histologically defined by “ependymoblastic rosettes.” Because it is so rare, little is known about specific MR imaging characteristics of ependymoblastoma. We systematically analyzed and discussed MR imaging features of ependymoblastoma in a series of 22 consecutive patients.MATERIALS AND METHODS:Ependymoblastoma cases were obtained from the database of the German multicenter HIT trials between 2002 and 2013. All cases within this study were centrally reviewed for histopathology, MR imaging findings, and multimodal therapy. For systematic analysis of initial MR imaging scans at diagnosis, we applied standardized criteria for reference image evaluation of pediatric brain tumors.RESULTS:Ependymoblastomas are large tumors with well-defined tumor margins, iso- to hyperintense signal on T2WI, and diffusion restriction. Contrast enhancement is variable, with a tendency to mild or moderate enhancement. Subarachnoid spread is common in ependymoblastoma but can be absent initially. There was a male preponderance (1.75:1 ratio) for ependymoblastoma in our cohort. Mean age at diagnosis was 2.1 years.CONCLUSIONS:With this study, we add the largest case collection to the limited published database of MR imaging findings in ependymoblastoma, together with epidemiologic data. However, future studies are needed to systematically compare MR imaging findings of ependymoblastoma with other CNS-primitive neuroectodermal tumors and ependymoma, to delineate imaging criteria that might help distinguish these pediatric brain tumor entities.

According to the 2007 World Health Organization Classification of Tumors of the CNS, ependymoblastoma (EBL) is a grade IV embryonal tumor that can be categorized as a subgroup of primitive neuroectodermal tumor (PNET).¹ The group of CNS-PNET can be further subdivided into CNS-neuroblastoma, CNS-ganglioneuroblastoma, NOS (not otherwise specified), and medulloepithelioma. EBLs are highly aggressive tumors that occur mainly in young children, with rapid growth and craniospinal dissemination. The MR imaging appearance of EBL has been described in the literature as a large, well-demarcated but heterogeneous mass with variable contrast enhancement.² Most of the tumors are located supratentorially, followed by infratentorial and spinal sites.³ Locations outside the CNS are exceptionally rare, with published cases of congenital sacrococcygeal or ovarian tumors.^4,5First described by Bailey and Cushing^6,7 in 1926 as an ependymal-derived entity, the exact definition of EBL has since generated some controversy among neuropathologists. Rubinstein⁸ later characterized EBLs as primitive neuroepithalial tumors of high cellularity that show numerous and characteristic “ependymoblastic rosettes.”We want to contribute to this ongoing discussion with the first systematic analysis of imaging characteristics of EBL. Differentiation from other primitive embryonal tumors (such as other CNS-PNET variants and medulloblastoma) by means of diagnostic imaging is challenging. Due to the rarity of this tumor, a systematic analysis of MR imaging features of EBL has not yet been performed. To determine specific diagnostic features, we report on imaging characteristics of 22 consecutive EBL cases that were collected from the prospective German Society of Pediatric Oncology and Hematology multicenter trials HIT91, HIT-SKK92, and HIT2000 (HIT is a German abbreviation for brain tumor), with central review for neuropathology, neuroradiology, and therapy.^3,9,10