Medulloblastoma

Look For First

Hyperdense midline mass in the roof of the fourth ventricle on CT or T1-hypointense/T2-hyperintense midline cerebellar mass on MRI
Effacement of the fourth ventricle with obstructive hydrocephalus and dilated lateral ventricles
High DWI signal with restricted diffusion (low ADC ~550 x 10-6 mm2/s) reflecting hypercellularity
Prominent heterogeneous enhancement on postcontrast imaging in 90% of cases

Key Image Findings

Medulloblastomas are extremely cellular small round blue cell tumors arising from the cerebellar vermis in 75% of cases and projecting into the fourth ventricle from its roof (superior medullary velum), distinguishing them from ependymomas which arise from the floor and squeeze the foramen of Luschka.
On CT, medulloblastomas appear hyperdense in 90% of cases; cyst formation and necrosis are common (40-50%), and calcification is seen in 10-20% of cases.
On MRI, tumors are T1-hypointense to grey matter with overall 90% enhancement that is often heterogeneous; T2/FLAIR show iso- to hyperintense signal with heterogeneity from calcification, necrosis, and cyst formation.
Restricted diffusion is the hallmark imaging feature, with high DWI signal and low ADC values (typically lower than normal cerebellum at ~550 x 10-6 mm2/s) due to their hypercellularity and high nucleus-to-cytoplasm ratio.
MR spectroscopy shows elevated choline, decreased NAA, and elevated taurine (3.4 ppm), with group 3/4 tumors showing high taurine and high creatine, while SHH tumors show little or no taurine and low creatine.
MRI can delineate the fourth ventricle and subarachnoid space better than CT; medulloblastomas project into the fourth ventricle but unlike ependymomas do not usually extend into the basal cisterns.
In 40% of patients at diagnosis, CSF seeding is present; therefore, contrast-enhanced imaging of the entire neuroaxis is recommended to identify drop metastases and leptomeningeal spread.
Location predicts molecular subgroup: cerebellar peduncle/foramen of Luschka indicates WNT-activated (best prognosis), cerebellar hemisphere indicates SHH subgroup (intermediate prognosis), and midline location may indicate group 3 or 4 tumors.

Differential Diagnosis

Ependymoma: typically arises from the floor of the fourth ventricle and squeezes out the foramen of Luschka, while medulloblastomas arise from the roof and project into the ventricle; ependymomas show less diffusion restriction.
Atypical teratoid/rhabdoid tumor (AT/RT): occurs in very young children and is highly aggressive; can be distinguished by age of presentation and associated syndromic features.
Pilocytic astrocytoma: characterized by a large cystic component with a brightly enhancing mural nodule, different from the heterogeneous enhancement of medulloblastoma.
Brainstem glioma (exophytic): typically arises from the brainstem itself rather than projecting into it from the cerebellum.
Choroid plexus papilloma: more common in the lateral ventricles in children and does not demonstrate the characteristic restricted diffusion of medulloblastoma.
Cerebellar metastasis (in adults): rare in children; adults with cerebellar lesions are more likely to have metastatic disease, though medulloblastoma remains a consideration in all ages.

Discussion

Medulloblastomas are now recognized as molecularly and histologically distinct subgroups (WNT-activated, SHH-activated, group 3, and group 4) with different prognoses, demographics, and imaging characteristics; WNT-activated tumors have very good prognosis while group 3 tumors have poor prognosis.

Peak incidence occurs between 3 and 7 years of age, but 14-30% of all medulloblastomas occur in adults, who typically present in the third and fourth decades with laterally located tumors and often better prognosis.

Imaging location is the key predictor of molecular subgroup: cerebellar peduncle suggests WNT-activated (excellent prognosis), hemispheric location suggests SHH (intermediate prognosis), and midline location suggests group 3 or 4 (variable prognosis based on enhancement and margin characteristics).

Prognosis is most strongly influenced by molecular subtype, but traditionally poor prognosis is associated with young age at diagnosis (<3 years), incomplete surgical resection (>1.5 cm3 residual), and presence of CSF metastases at diagnosis.

Restricted diffusion with low ADC values is a hallmark feature reflecting the extremely high cellularity and high nucleus-to-cytoplasm ratio characteristic of these small round blue cell tumors.

Cerebellar mutism syndrome may occur as a complication after surgical resection, emphasizing the importance of preoperative imaging and surgical planning to minimize morbidity.

Reporting Pearls

Describe the location precisely (midline vermis vs. hemispheric vs. cerebellar peduncle), measure any residual disease after resection, document the degree and pattern of enhancement (homogeneous vs. heterogeneous), assess for restricted diffusion, evaluate for CSF seeding in the entire neuroaxis, and note any hemorrhage which may raise suspicion for group 4 tumors in adults.

Pitfalls

Confusing medulloblastoma with ependymoma: remember that medulloblastomas arise from the roof of the fourth ventricle and project inward, while ependymomas arise from the floor and squeeze the foramen of Luschka.
Underestimating the frequency of CSF seeding: approximately 40% of medulloblastomas have CSF metastases at diagnosis, requiring contrast-enhanced imaging of the entire neuroaxis for complete staging.
Failing to recognize that medulloblastoma location predicts molecular subgroup and prognosis: lateral cerebellar hemisphere location favors SHH (intermediate prognosis), while cerebellar peduncle location strongly suggests WNT-activated (excellent prognosis).
Overlooking group 4 tumors which may show less enhancement than pediatric medulloblastomas; in older children and adults, look for well-defined midline tumors with mild or no enhancement as a sign of group 4 biology.

Medulloblastoma

Key Images

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