ATRT

​IMAGING OF PRIMARY POSTERIOR FOSSA BRAIN TUMORS IN CHILDen

INTRODUCTION

Brain tumors represent the most common solid neoplasm in children and second most common pediatric malignancy overall, following leukemia. Outside of infancy, the majority of primary childhood brain tumors occur in the infratentorial compartment and include medulloblastoma, juvenile pilocytic astrocytoma (JPA), ependymoma, and brainstem/pontine glioma; atypical teratoid rhabdoid tumor (ATRT) is an additional rare but important primary brain tumor of early childhood.

Children with posterior fossa brain tumors typically present with signs and symptoms related to increased intracranial pressure, gait disturbances, and/or cranial nerve deficits, depending upon the type, size, and location of the tumor. CT and MR evaluation is critical in the work-up, management, and follow-up of these patients. This article aims to provide an overview of the imaging manifestations and appearances of the most common primary posterior fossa brain tumors in children.

MEDULLOBLASTOMA

Background.

Medulloblastoma is a highly malignant (grade IV) embryonal cerebellar tumor which occurs most frequently in children but may also affect adults. It is the second most common pediatric brain tumor overall, following astrocytoma, but is the most common pediatric posterior fossa tumor, accounting for up to 40% of cases.1-5 In the pediatric population, there are two age peaks – one at 3 and one at 7 years of age;6 in adults, the peak age of presentation is between 20 and 40 years of age.3,7,8 Boys are affected more often than girls by a ratio of greater than 2:1. There is an association and increased incidence in the setting of basal cell nevus, or Gorlin, syndrome.

Medulloblastomas are categorized into various pathologic subtypes, including classic, desmoplastic, extensively nodular, large cell, and anaplastic.3,9 The classic subtype is by far the most common. An additional rare subtype is the melanotic medulloblastoma, which appears similar to a classic medulloblastoma on imaging but has melanotic tint on gross inspection. The vast majority of medulloblastomas (85-90%) arise from the midline cerebellar vermis dorsal to the fourth ventricle.10 Approximately 10-15% occur within the cerebellar hemispheres, which is more common in older patients with the desmoplastic variant.1 More aggressive variants may be infiltrative and invade the fourth ventricle and adjacent brainstem or cerebellar parenchyma.

Children with medulloblastomas most often present clinically with a relatively rapid onset of symptoms - over the course of weeks or a few months - due to the rapid growth and malignant features of the neoplasm. The most common symptoms include headaches and nausea/vomiting due to obstructive hydrocephalus; truncal ataxia and papilledema are common clinical findings. Tumoral seeding of the cerebral spinal fluid (CSF) is present in approximately one-third of cases at the time of initial diagnosis.

Imaging Findings.

On CT, medulloblastomas typically present as well-defined, midline posterior fossa masses. The high cellularity with increased nuclear-to-cytoplasmic ratio leads to increased attenuation on unenhanced CT in approximately 90% of cases (Fig. 1); the remainder are isodense to brain parenchyma.3 Surrounding parenchymal vasogenic edema is noted in >90% of cases. After contrast administration, there is often avid enhancement which may be homogeneous or heterogeneous. Calcifications are seen in approximately 20% of cases; cysts are more common and occur in 50-60% of cases.11 As the lesions grow, there is anterior displacement and compression of the fourth ventricle, which often leads to obstructive hydrocephalus in approximately 90% of cases.12 If uncompensated, transependymal flow of CSF may be seen along the margins of the lateral ventricles.    

On MRI, the majority of medulloblastomas are iso- to hypointense compared to white matter on T1 sequences and variable in signal on T2 sequences.3 The T2 signal variability has to do with the cellularity of the tumor. More cellular components are hypointense, while less cellular components are iso- to mildly hyperintense compared to white matter. Increased cellularity also leads to increased signal intensity on diffusion weighted sequences; although helpful, this finding has some overlap with other posterior fossa tumors .13 Medulloblastomas commonly demonstrate avid but heterogeneous enhancement on MRI (Fig. 2). Regions of CSF dissemination present with focal (more common) or diffuse leptomeningeal enhancement, which is typically nodular (Fig. 3). MR spectroscopy demonstrates a tumoral spectra with increased choline and decreased N-acetyl aspartate (NAA), along with decreased creatine. Lipid-lactate doublets may also be seen. An elevated taurine peak has recently been shown to be a specific MRS finding for medulloblastoma (Fig. 4).14-15

Medulloblastomas grow in a circunferential pattern and maintain rounded borders. Unlike ependymomas, which are soft and pliable, medulloblastomas infrequently extend through CSF outlet foramina. When aggressive, however, medulloblastomas may be more infiltrative and invade the fourth ventricle, brainstem, and/or adjacent cerebellar parenchyma.

Fig. 2

Fig. 3

Fig. 4

Preoperatively, it is critical to evaluate the entire neuroaxis on MRI to look for disseminated disease (Fig. 5). Failure to do so could complicate management decisions. Postoperatively, surveillance imaging of the brain and spine is typically performed at 3-6 months intervals for the first 5 years following initial therapy to evaluate for early recurrence or new CSF dissemination.3 Although the long-term impact of surveillance imaging on overall survival is debated, early detection of new or recurrent disease may alter clinical management.

  Fig. 5