Brain tumors are the most frequent solid neoplasm in children and the first cause of mortality in infancy.
Having a pathological spectrum completely different to the adult one,
the treatment of pediatric brain tumors presents many difficulties because of the nature of the developing brain.
The localisation of the tumors according to patient age is as follow:
- 0 to 3 years old (yo) : supratentorial > infratentorial
- 4 to 10 yo : infratentorial > supratentorial
- 10 to early adult hood : infratentorial = supratentorial
- adults : supratentorial > infratentorial
We will discuss here about infratentorial tumors of posterior fossa in pediatric age (until 18 yo).
Conventional MRI is an essential tool for diagnosis and evaluation of location and extent of posterior fossa tumors,
but offers limited information regarding tumor grade and type.
Advanced MRI techniques such as diffusion weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps may improve the specific diagnosis of these tumors in children.
Herniation,
brainstem compression and death are all risks in tumors which occur in this critical location.
Because these various tumors require different treatment and have different natural histories and outcomes,
an accurate and specific diagnosis is mandatory.
For each described tumor there is a short checklist that every young radiologist should keep in mind.
Pilocytic astrocytoma
Definition: well circumscribed tumor,
often cystic,
slow growing.
Best diagnostic clue: cystic cerebellar mass with enhancing mural nodule; enlarged optic/ chiasm/ tract with variable enhancement.
Imaging findings:
On CT the lesion consist of mixed cystic/ solid mass with variable edema.
The solid component is hypodense to gray matter (GM) and often the astrocytoma causes obstructive hydrocephalus.
After contrast administration,
the solid nodule enhance hetero-/ homegeneous.
On T1-weighted images (T1WI),
the solid portion is iso/ hypointence to GM and the cyst contents is iso- to slightly hyperintense to cerebrospinal fluid (CSF).
On T2-weighted images (T2WI),
the solid portion is hyperintense to GM and the cyst contents is iso- to slightly hyperintense to CSF (Figure 1 and 2).
Diffusion weighted imaging (DWI) identifies a similar diffusivity to GM of the solid nodule.
After contrast administration,
the solid portion shows intense but heteroogeneous enhancement and the cyst wall occasionally enhance,
thus suggesting tumoral aspect of the cyst.
There can be found rarely leptomeningeal metastases.
Giving these aspects,
the best imaging tool is contrast-enhanced MRI.
Medulloblastoma
Definition: It is a malignant,
invasive,
highly cellular embryonal tumor
Best diagnostic clue: round,
dense 4th ventricle mass arising from roof of 4th ventricle
Imaging findings:
On CT the tumor is hyperdense and may present in 20-50% of cases intratumoral cyst or necrosis.
On the other hand,
in 95% of cases the tumor causes hydrocephalus.
After contrast administration it usually enhance in a homogeneous way.
On T1WI the tumor is hypointense to GM and the 3dr ventricle is dilated.
On T2WI,
the signal is near GM intensity and there is a rim of CSF on one or more sides of the mass (Figure 3 and 4).
DWI reveals a reduced diffusion of the tumor compared to parenchyma,
aspect being characterized by a high degree of cellularity and often containing cells with a high nuclear-cytoplasmic ratio,
which provides an increased number of membrane barriers to microscopic water diffusion.
Two different types of enhancement may be present:
- Zuckerguss- linear "sugar icing" - like enhancement over brain surface;
- Grape-like tumor nodules,
which are less common.
Again the best imaging tool is contrast-enhanced MRI.
Infratentorial ependymoma:
Definition: slow growing tumor of ependymal cells
Best diagnostic clue: soft tumor that squeezes out through the 4th ventricle foramen into the cistens.
It arises from the floor of the 4th ventricle.
Imaging findings:
On CT the tumor is heterogeneous and may present cyst,
calcification or hemorrhage.
This tumor also commonly causes hydrocephalus.
After contrast administration it enhances in a heterogeneous way.
On MR,
first on T1WI the tumor is iso- to hypointense to GM and cystic foci are slightly hyperintense to CSF.
On T2WI,
the tumor is hyperintense and can't make the difference between tumor,
calcified and hemorrhagic foci (Figure 5 and 6).
DWI identifies the cystic or necrotic components which have restricted diffusion.
After contrast administration the tumor presents a mild to moderate heterogeneous enhancement.
Once more the best imaging tool is contrast-enhanced MRI.
Brainstem glioma:
Definition: distinguished by location and histology characteristics,
brainstem gliomas usually affect the pons.
Best diagnostic clue: non-enhancing expansive mass into pons that produce hydrocephalus
Imaging findings:
On CT the tumors present decreased attenuation and enlargement of the affected region.
The pontine tumors are flattering the anterior border of 4th ventricle.
There is mild to absent enhancement of the tumors.
At MR,
on T1WI the tumors are mild to moderately hypointense to GM.
Central areas of preserved signal may reflect unaffected white-matter (WM) tracts.
On T2WI,
the tumor has bright signal and the difference between edema and infiltrating tumor can be identify on this sequence.
Sometimes the exophytic component of the tumor can engulf the basilary and vertebral arteries.
Most infiltrating gliomas don't have reduced diffusion on DWI,
but foci of reduced diffusion may reflect necrosis or higher grade.
If the tumor presents enhancement after contrast administration,
this suggests a tumor of higher grade.
In case of glioblastomas there is rim-enhancement (Figure 7).
Nevertheless the best imaging tool is contrast-enhanced MRI.
DWI might,
in theory,
effectively distinguish between tumor types and histologic grades,
as higher grade tumors have more densely packed cells that should have increasingly restricted diffusion (with a lower ADC).
Checklist for differential diagnosis:
Medulloblastoma - hyperdense midline mass that fills the 4th ventricle
- younger patient age group of 2-6 years old
Ependymoma - plastic tumor that extends through 4th ventricle foramem
- calcified cysts,
hemorrhage common
- heterogeneous enhancement
Cerebellar pilocytic astrocytoma - older children
- hemispheric lesion
- cyst with vigorously enhancing nodule
Brain stem glioma - infiltrating mass expanding brainstem
- may project into 4th ventricle
Other:
- choroid plexus papilloma - much less common in 4th ventricle
- less mass effect
- avidly enhancing 4th ventricular mass in an infant, and in lateral ventricle in children
- atypical teratoid rhabdoid tumor - large mass with cyst or necrosis - variable enhancement pattern - younger children
- hemangioblastoma - large cyst with small enhancing mural nodule - tumor in adults - associated with von Hippel Lindau disease (Figure 8)