◊Most malignant infratentorial neoplasm; most common neoplasm of posterior fossa in childhood (followed by cerebellar pilocytic astrocytoma)

Incidence: 15–20% of all pediatric intracranial tumors; 30–40% of all posterior fossa neoplasms in children; 2–10% of all intracranial gliomas

Origin: external granular layer of inferior medullary velum (= roof of 4^th ventricle)

Histo: completely undifferentiated cells (50%), desmoplastic variety (25%), glial / neuronal differentiation (25%)

Age: 40% within first 5 years of life; 75% in first decade; between ages 5 and 14 (⅔); between ages 15 and 35 (⅓); M÷F = 2–4÷1

• duration of symptoms <1 month prior to diagnosis: nausea, vomiting, headache, increasing head size, ataxia

Site:

1. vermis cerebelli + roof of 4^th ventricle (younger age group) in 91%
2. cerebellar hemisphere (older age group)

Size: usually >2 cm in diameter

• well-defined vermian mass with widening of space between cerebellar tonsils
• encroachment on 4^th ventricle / aqueduct with hydrocephalus (85–95%)
• shift / invagination of 4^th ventricle
• rapid growth with extension into cerebellar hemisphere / brainstem (more often in adults)
• extension into cisterna magna + upper cervical cord, occasionally through foramina of Luschka into cerebellopontine angle cistern
• mild / moderate surrounding edema (90%)

CT:

Classic tumor features in 53%:

• slightly hyperdense (70%) / isodense (20%) / mixed (10%)
• rapid intense homogeneous enhancement (97%) ← tumor usually solid

Atypical features:

• cystic / necrotic areas (10–16%) with lack of enhancement
• calcifications in 13%
• hemorrhage in 3%
• supratentorial extension

MR:

• mixed / hypointense on T1WI
• hypo- / iso- / hyperintense on T2WI
• usually homogeneous Gd-DTPA enhancement with hypointense rim
• cerebellar folia blurred

Cx:

1. Subarachnoid metastatic spread (30–100%) via CSF pathway to spinal cord + cauda equina (“drop metastases” in 40%), cerebral convexities, sylvian fissure, suprasellar cistern, retrograde into lateral + 3^rd ventricle
   • continuous “frosting” of tumor on pia
2. Metastases outside CNS (axial skeleton, lymph nodes, lung) after surgery

Rx: surgery + radiation therapy (extremely radiosensitive)

DDx of midline medulloblastoma:

ependymoma, astrocytoma (hypodense)

DDx of eccentric medulloblastoma:

astrocytoma, meningioma, acoustic neuroma