Cerebellar pilocytic astrocytoma in a pediatric patient

A 3-year-old male patient presented with instability, headache and vomiting episodes that had started nearly a month before and progressively worsened. Neurological examination revealed signs of cerebellar disfunction (ataxia, gait disturbance and hypotonia) and increased intracranial pressure (the fundus oculi examination revealed bilateral papilloedema).

Non-enhanced CT (NECT) showed a large mass in the posterior cerebral fossa near the IV ventricle, arising from the cerebellar vermis, with two cyst-like masses on the superior side, obstructing the III ventricle and the sylvian aqueduct with resulting hydrocephalus (fig.1).

Multiple MRI sequences have been obtained to better assess the mass nature and extent.

On T2W, the lesion appears heterogeneously hyperintense, with inner microcystic components; the cystic masses appear hyperintense to CSF. There is a halo of high signal intensity around the dilatated lateral ventricles, standing for transependymal oedema (fig. 2).

On FLAIR, cystic components appear hyperintense compared to CSF, whose signal is suppressed. The solid nodule is partially hyperintense to grey matter (fig. 3).

DWI demonstrates little restriction of diffusion in the solid part while there is no restriction in cystic parts. (fig. 4).

On post-contrast T1W, the lesion presents an inhomogeneous pattern of contrast-enhancement, due to its solid-cystic structure (fig. 5).

Pilocytic astrocytoma (PA) represents the most frequent brain tumour in children aged 0 to 14 years [1], [2].

It is classified as a grade I astrocytoma according to the WHO classification of tumours of the central nervous system (CNS) [3]. The more usually involved regions are the cerebellum, the optic nerve and chiasma, and the hypothalamus [4].

At pathological examination, PAs are generally described as well-defined slow-growing cystic tumours [5] that tend to displace and compress the adjacent parenchyma rather than infiltrate it. Despite their usual behaviour, parenchymal invasion and leptomeningeal spread occasionally occur.

The clinical presentation of PAs varies upon the anatomical regions involved in the CNS and the age of the patient at the time of diagnosis.

Posterior fossa tumours usually manifest clinical signs of cerebellar dysfunction. Compression of the IV ventricle with associated hydrocephalus occurs later in the course of disease and causes signs and symptoms of increased intracranial pressure.

Diagnostic imaging is essential for localization and assessment of tumour extent prior to treatment and also for follow-up [6].

On CT and MRI PA typically presents as a well-demarcated, contrast-enhancing lesion composed by both solid and micro- or macro-cystic components. A cyst with an intensely enhancing mural nodule is the most common presentation (50%) [7], and is more typical of posterior fossa Pas [8]. Cysts and central necrotic areas cause the enhancement pattern to be mostly heterogeneous, while the solid portions tend to enhance homogeneously [9].

CT scans show a mixed cystic-solid or solid mass causing parenchymal compression; oedema of the surrounding brain is infrequently found. The solid component is usually iso-hypodense.

On MRI, the solid component appears hypo- or isointense on T1-weighted images and hyperintense on T2-weighted and FLAIR images compared to grey matter [10]. The cystic components appear slightly hyperintense compared to CSF on both T1 and T2 weighted images; they are only partially suppressed on FLAIR images [11].

On DWI, cystic components have the same diffusion properties as CSF while the solid component shows high apparent diffusion coefficient values, due to the low cellularity of PA [12].

In some cases, imaging studies can demonstrate unusual findings, including tumour calcifications (10-20%), small cyst formations, heterogeneously or irregularly enhancing tumour nodules and internal haemorrhages [13].

PA is an indolent tumour with a good overall survival (OS), especially in pediatric patients (10-year OS > 90%) [1].

Tumours located in anatomical regions that allow complete resection (e.g. cerebellum) have a better OS [14].

Written informed patient consent for publication has been obtained.