ASTROCYTOMA

• Frequency: most common posterior fossa tumors in children(30–35%).
• Age and Gender: Peak incidence 5–13 years, with equal gender frequency.
• Genetic Associations: Linked to NF1, Turcot syndrome, PHACE(S), Ollier’s disease.
• Clinical Presentation: Long history of fluctuating signs of increased intracranial pressure. Unilateral or asymmetric cerebellar signs if the tumor extends into one hemisphere.

1. Juvenile Pilocytic Astrocytomas (JPA)

• Frequency: one of the most frequent tumors in the posterior fossa in children.
• Biology: Typically benign, low-grade (WHO I), and slow-growing.
• Prognosis: Excellent, with a 25-year survival rate close to 90%.
• Imaging Findings: (Fig.1)

• CT: Predominantly cystic mass with a mural nodule; solid component is hypodense.
• MRI:
• Variable, with cystic (50%), solid with necrosis (40%), or solid (10%) forms
• T1-weighted: solid component hypo- to isointense compared to gray matter. Homogeneous or heterogeneous intensity.
• T2-weighted: solid components are hyperintense.
• Enhancement: Variable, usually heterogeneous. Solid portions enhance prominently and homogeneously.
• Hemorrhages: Rare and typically not massive.
• Treatment: Surgical resection.

2. High-Grade Cerebellar Astrocytomas

• Frequency: 10% of all pediatric high-grade gliomas.
• Genetic Associations: Turcot syndrome, Ollier’s disease.
• Age and Clinical Presentation: More common in older children. Symptoms develop more rapidly due to their higher growth rate. Typical symptoms include increased intracranial pressure and cerebellar ataxia.
• Imaging Findings:
• More heterogeneous than JPAs, with prominent, heterogeneous, ring-like enhancement and poorly defined margins.
• Multicentricity, extra-axial metastases, and larger size are frequent.
• Leptomeningeal dissemination: Rare, but possible.
• Additional Evaluation: Requires full neuroimaging of the neuroaxis.

MEDULLOBLASTOMA

• Frequency: one of the most common malignant pediatric tumors in the posterior fossa. (20% of all brain tumors).
• Age and Gender: 3–8 years, higher incidence in boys.
• Clinical Presentation: typically related to obstructive hydrocephalus, such as headaches, vomiting, and cerebellar ataxia. Cerebellar signs (incoordination, gait disturbances) are common.
• Imaging Features: (Fig.2.)
  • CT Imaging: Solid, hyperdense mass, often with calcifications (30% of cases) mostly in the inferior vermis.
  • MRI Imaging:
    • Round, slightly lobulated T1-iso-/hypointense mass. The T2 signal is heterogeneous and the solid portion is often T2- hypo- to isointense compared with the gray matter.  Cysts and calcifications.
    • Contrast enhancement is usually present, but may be variable.
    • Dissemination to the CSF is common seen in T1 sequences after contrast. Drop metastases may present in the spinal cord.
  • Prognosis: Depends on extent of resection, age, metastases, and tumor subtype. Survival rates vary between 50% and 80%, with better outcomes for younger patients and localized tumors.
  • Treatment: Combination of surgical resection, chemotherapy, and radiotherapy, with focus on reducing metastasis and improving survival.

EPENDYMOMA

• Frequency: forth most common. 5–10% of pediatric brain tumors.
• Age and Gender: children under 5 years, slight male predominance.
• Clinical Presentation:  related to obstructive hydrocephalus, including nausea, vomiting, and headaches.
• Imaging Features:
  • CT Imaging: Solid mass, typically hyperdense within the IV ventricle, with or without punctuate calcifications.
  • MRI Imaging: (Fig.3.)
    • Homogenous mass filling and distending IV ventricle.
    • T1- iso/hyporintense and T2-hypointense. Heterogeneity can occur due to cysts and mixoid components.
    • Enhancement is often heterogeneous.
    • The tumor may be associated with CSF dissemination.
  • Prognosis: depends on resection and CSF dissemination.
  • Treatment: Surgical resection may be difficult if formaminal extension.

BRAINSTEM GLIOMA

1. Diffuse Intrinsic Brainstem Gliomas (DIPG).

• Frequency: These are among the most aggressive brainstem tumors, children between 5-9 years. 10-15% of all pediatric brain tumors.
• Clinical Presentation: related to cranial nerve involvement and progressive neurologic deficits.
• Imaging Features:
  • CT Imaging: Hypodense mass centered in an expanded pons, with cystic component occasionally.
  • MRI Imaging: (Fig.4 y 5)
    • T1-weighted: Hypointense compared to normal brain tissue.
    • T2-weighted: Heterogeneously ill-defined hyperintense areas are often seen throughout the brainstem, reflecting the infiltrative nature of the tumor. Usually larger than 2 cm. Anterior extension typically ‘‘embraces’’ the basilary artery.
    • Enhancement: No or minimal enhancement.
  • Prognosis: Extremely poor. 9-12 months survival after diagnosis.
  • Treatment: Due to the location, they are not surgical approachable. In children younger than 3 years, RT is also limited as brain is immature and too vulnerable.

2. Exophytic Gliomas of the Lower Brainstem/Medulla Oblongata.

• Frequency: less common than DIPGs, primarily affecting younger children.
• Clinical Presentation: cerebellar signs, such as ataxia, and signs of increased intracranial pressure (e.g., vomiting, headache). These tumors can present with cranial nerve involvement, particularly those arising from the medulla.
• Imaging Features:
  • CT Imaging: Exophytic hipo or isodense mass in lower brainstem.
  • MRI Imaging:
    • T1-weighted: Hypointense.
    • T2-weighted: Hyperintense, with well-defined borders. Extend ‘‘exophytically’’ into IV ventricle or perimedullary cisterns rather than infiltrate the adjacent brainstem
    • Enhancement: homogeneously or heterogeneously.
  • Prognosis: is better than for DIPGs.
  • Treatment: Surgical resection is the treatment of choice.

3. Tectal Gliomas

• Frequency: rare type of glioma found in the tectal plate (the area in the midbrain). Children aged 5 to 10 years.
• Clinical Presentation: Symptoms usually include hydrocephalus due to obstruction of the cerebral aqueduct, as well as signs of brainstem compression (headache, vomiting, and ataxia). Cranial nerve deficits may also be seen.
• Imaging Features:
  • CT Imaging: small, nonenhacing, hypodense mass lesions centered in the tectal plate.
  • MRI Imaging:
    • T1-weighted: Hypointense to brain tissue.
    • T2-weighted: Hyperintense lesions are seen, often with well-defined margins.
    • Enhancement: variable, usually homogeneous.
  • Prognosis: Relatively favorable, especially when slow-growing and localized.
  • Treatment: Surgery can be challenging due to the tumor's deep location, in many cases, they are left for observation. Radiation therapy may be used if symptoms worsen or if there is progression of the tumor.

HEMANGIOBLASTOMA

• Frequency: rare and benign.
• Genetic associations: von Hippel-Lindau (VHL) syndrome.
• Clinical Presentation: due to cystic expansion causing compression of adjacent structures, leading to hydrocephalus or cerebellar signs.
• Imaging Features:
  • CT Imaging: Well-defined cystic lesion with an mural solid nodule.
  • MRI Imaging:
    • T1 -hyperintensity if recent intracystic hemorrage. T1-hypointense.
    • Hyperintense on T2-weighted images.
    • Enhancement: strong homogenous solid nodule enhancement.
    • MR angiography: high vascularity. Prominent and serpiginous supplying vessels may be seen.
  • Prognosis: Prognosis is generally good if the tumor can be surgically resected. Tumors in the context of VHL syndrome require careful monitoring for recurrence.
  • Treatment: Surgical resection.

ATYPICAL TERATOID/RHABDOID TUMOR (ATRT)

• Frequency: rare and highly malignant tumor.
• Age and Gender: very young (less than 2 years)
• Clinical Presentation: rapid-onset increased intracranial pressure, vomiting, and developmental regression.
• Imaging Features:
• CT Imaging: Ill-defined, heterogeneous mass with possible areas of calcification.
• MRI Imaging: Heterogeneous appearance with mixed solid, necrotic and cystic components. Hyperintense on T2-weighted imaging.
• Prognosis: Poor prognosis, often metastasizes. The survival rate is low.
• Treatment: Treatment involves aggressive surgery, chemotherapy, and radiotherapy, but outcomes remain dismal, especially in cases with metastasis.

SCHWANNOMA

• Frequency: rare. 2% of posterior fossa tymors.
• Genetic associations: NF2 (often bilateral).
• Imaging Features:
• CT Imaging: hypo- to isodense, may calcify and may show a mild to strong contrast enhancement.
• MRI Imaging: centered in cerebeollo-pontine angle, well circumscribed. T1-hipointense and T2-hyperintense. homogeneous or heterogeneous, strong contrast enhancement.

EPIDERMOID TUMOR

• Frequency: congenital, benign.
• Clinical Presentation: cranial neuropathies or hydrocephalus.
• Imaging Features:
• CT Imaging: hypodense and lobulated mass. 50% in cerebellopontine angle.
• MRI Imaging: T1-hypointense, and T2 hypo- to hyperintense presenting a signal density and intensity similar to that of the CSF. On FLAIR, epidermoid tumors are often slightly hyperintense. No enhancement.

CEREBELLAR GANGLIOCYTOMA

• Frequency: rare, WHO grade 1 tumor.
• Genetic associations: Cowden syndrome.
• Imaging Features:
• CT Imaging: diffuse increase in volume of cerebellar folia.
• MRI Imaging: the inner portions of the folia are T1 hypointense and T2 hyperintense, while the outer portions are T1 isointense and T2 iso- to hypointense.

METASTASES

• Frequency: rare. 2-6% incidence.
• Associations: most commonly result from neuroblastoma, Ewings’s sarcoma, osteogenic sarcoma, rhabdomyosarcoma, and Wilm’s tumors.
• Imaging Features:
  • Variable in size and morphology, enhancement.
  • Significant amount of vasogenic edema.