# 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

| Tumor | Age | Location | DWI | Enhancement | Key Feature |
|-------|-----|----------|-----|-------------|-------------|
| JPA | 5-15 yrs | Cerebellar hemisphere | No restriction | Mural nodule enhances | Cyst + enhancing nodule; WHO Grade I |
| Medulloblastoma | 5-10 yrs | Midline vermis/4th ventricle | Restricted (low ADC) | Heterogeneous | Hyperdense on CT; CSF drop mets |
| Ependymoma | 1-5 yrs | Floor of 4th ventricle | No restriction | Irregular | Extends through foramina of Luschka |
| AT/RT | <3 yrs | Posterior fossa | Restricted | Heterogeneous | INI1 loss; mimics medulloblastoma |
| DIPG | 5-10 yrs | Pons | Variable | Minimal/none | Expansile; engulfs basilar artery |

### 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.