Residency · Residency · Diagnostic Radiology

Pediatric Brain Tumors and Posterior Fossa Masses

Introduction

Brain tumors are the most common solid tumors in children and the second most common pediatric malignancy after leukemia. Unlike adults, where supratentorial tumors predominate, posterior fossa tumors account for approximately 60% of pediatric brain tumors. MRI is the primary imaging modality for characterization, staging, and surveillance.

General Principles

The peak incidence of pediatric brain tumors falls between 5 and 10 years of age. Posterior fossa tumors predominate in children aged 1-10, while supratentorial tumors are more common in infants and adolescents. Common presenting symptoms include headache, vomiting, and ataxia due to obstructive hydrocephalus. The MRI protocol should include T1, T2, FLAIR, DWI, post-contrast T1, and MR spectroscopy when available. Spinal MRI should be obtained pre-operatively for tumors prone to CSF dissemination, particularly medulloblastoma and ependymoma.

Posterior Fossa Tumors

Juvenile Pilocytic Astrocytoma (JPA)

JPA is the most common pediatric posterior fossa tumor. Its classic appearance is a cystic mass with an enhancing mural nodule arising from the cerebellar hemisphere. The cyst wall typically does not enhance, which helps distinguish it from hemangioblastoma in adults. It is a WHO Grade I tumor with an excellent prognosis after surgical resection. Low ADC values are uncommon, and MR spectroscopy shows elevated choline and low NAA. When located in the optic pathway, it is associated with neurofibromatosis type 1 (NF1).

Medulloblastoma

Medulloblastoma is the most common malignant brain tumor in children. It arises from the cerebellar vermis in the midline, projecting into the fourth ventricle. It is typically hyperdense on CT due to high cellularity and a high nuclear-to-cytoplasmic ratio. Restricted diffusion on DWI (low ADC) is a hallmark finding. Enhancement is heterogeneous with areas of necrosis and calcification. CSF dissemination (drop metastases) occurs in up to 30% at diagnosis, necessitating craniospinal MRI staging. Four molecular subgroups (WNT, SHH, Group 3, and Group 4) have been identified with varying prognoses.

Ependymoma

Ependymoma is the third most common posterior fossa tumor in children. It arises from the floor of the fourth ventricle and characteristically extends through the foramina of Luschka into the cerebellopontine angle cisterns, earning its description as a "plastic" tumor. Signal is heterogeneous on MRI with calcification, cystic change, and hemorrhage. Importantly, it does not typically restrict diffusion, which helps distinguish it from medulloblastoma. Enhancement is irregular. While the rate of CSF dissemination is lower than medulloblastoma, local recurrence is common.

Atypical Teratoid/Rhabdoid Tumor (AT/RT)

AT/RT is a highly aggressive tumor occurring primarily in children under 3 years of age. It often mimics medulloblastoma on imaging with restricted diffusion and heterogeneous enhancement. It is associated with loss of INI1 (SMARCB1) gene expression and carries a very poor prognosis compared to medulloblastoma. It may contain hemorrhage, necrosis, and calcification.

Posterior Fossa Tumor Comparison

Brainstem Glioma

Diffuse intrinsic pontine glioma (DIPG) is the most common brainstem glioma. It appears as an expansile T2-hyperintense mass centered in the pons, engulfing the basilar artery, with minimal or no contrast enhancement. Diagnosis is typically made on imaging alone without biopsy. The prognosis is extremely poor, with median survival less than 12 months.

Supratentorial Tumors

Craniopharyngioma

Craniopharyngioma is the most common suprasellar tumor in children, with the adamantinomatous type predominating in the pediatric population. The classic triad is calcification, cystic components, and solid enhancing tissue. CT demonstrates calcification in over 90% of cases. MRI shows T1-hyperintense cyst content due to cholesterol and protein. It presents with visual field deficits and endocrine dysfunction.

Optic Pathway Glioma

Most optic pathway gliomas are juvenile pilocytic astrocytomas (WHO Grade I). There is a strong association with NF1, and bilateral optic nerve gliomas are pathognomonic for this condition. MRI shows fusiform enlargement and kinking of the optic nerve, and the tumor may extend along the optic chiasm, tracts, and radiations. These tumors are often followed with serial MRI rather than biopsied.

Supratentorial Ependymoma and PNET

Supratentorial ependymomas tend to be parenchymal rather than intraventricular, appearing as large, heterogeneous, cystic masses with calcification. Supratentorial PNETs are now reclassified under various molecular subtypes.

Advanced Imaging Techniques

MR Spectroscopy

An elevated choline-to-NAA ratio is seen in high-grade tumors, reflecting increased cellular turnover. Elevated lactate suggests anaerobic metabolism and necrosis. A taurine peak may be specific to medulloblastoma. Spectroscopy helps differentiate tumor from non-neoplastic processes.

Perfusion Imaging

Elevated relative cerebral blood volume (rCBV) correlates with higher tumor grade and is useful for monitoring treatment response and differentiating recurrence from radiation necrosis.

Clinical Pearls

Posterior fossa tumors account for the majority of brain tumors in school-age children, with the key differential including JPA, medulloblastoma, and ependymoma. Restricted diffusion (low ADC) favors medulloblastoma over JPA or ependymoma. Ependymoma characteristically extends through the foramina of Luschka and is referred to as the "plastic" tumor. DIPG is diagnosed on imaging alone, and biopsy is generally not performed. Always obtain craniospinal MRI for medulloblastoma to evaluate for CSF drop metastases.

References

1. Pediatric Brain Tumors: An Update on Imaging. Radiographics. 2018;38(4):1170-1188.
2. Posterior Fossa Tumors in Children: An Approach to Diagnosis. Pediatr Radiol. 2019;49(10):1353-1370.
3. Louis DN, et al. The 2021 WHO Classification of Tumors of the Central Nervous System. Acta Neuropathol. 2021;142(2):185-213.
4. Advanced MR Imaging Techniques in Pediatric Neuro-Oncology. Neuroimaging Clin N Am. 2017;27(1):167-185.

For the intern

At 3am, obtain non-contrast head CT to assess for acute hydrocephalus and mass effect—this determines if the child needs urgent neurosurgical intervention. Check if the patient is stable and symptomatic (altered mental status, respiratory changes, pupillary abnormalities). If unstable with signs of herniation, call neurosurgery immediately. Comprehensive MRI with gadolinium, diffusion-weighted imaging (DWI), and spectroscopy can wait until morning when radiology and neurosurgery teams are fully available. For a stable child with headache and vomiting, IV fluids and antiemetics can temporize overnight. Document neurologic exam carefully. Pre-operative spinal imaging for suspected medulloblastoma or ependymoma should be coordinated with neurosurgery but is not a 3am emergency.

Common pitfalls

• Forgetting to check ADC values: Restricted diffusion (low ADC) strongly suggests medulloblastoma or atypical teratoid/rhabdoid tumor (AT/RT), whereas juvenile pilocytic astrocytoma (JPA) and ependymoma typically show facilitated diffusion. Missing this can delay appropriate surgical planning and affect molecular testing protocols.

• Not obtaining pre-operative spine MRI: Medulloblastoma and ependymoma disseminate through cerebrospinal fluid (CSF). Post-operative blood products obscure drop metastases, so staging must occur before surgery.

• Confusing location cues: Medulloblastoma arises from the vermis (midline), ependymoma from the fourth ventricle floor (extends through foramina of Luschka), and JPA from the cerebellar hemisphere (lateral). Getting the origin wrong leads to incorrect differential diagnosis.

• Overreliance on enhancement patterns: All three common posterior fossa tumors enhance, but patterns overlap significantly. Combine location, patient age, and DWI findings for accurate diagnosis.

<image> Create a simple flowchart-style decision algorithm for pediatric posterior fossa masses. Panel 1: Start with "Posterior Fossa Mass" at top. Panel 2: First branch asks "Patient age <3 years?" with arrow to "Consider AT/RT" if yes. Panel 3: Main pathway continues to "Check DWI/ADC," branching into "Restricted diffusion" (arrow to "Medulloblastoma likely - midline vermis location") versus "No restriction" (arrow splitting two ways). Panel 4: The "No restriction" branch asks "Location?" with one arrow pointing to "Cerebellar hemisphere + cyst with nodule = JPA" and another to "4th ventricle floor + plastic/extends through foramina = Ependymoma." Use simple boxes and directional arrows; keep text minimal within each box. </image>

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