Medulloblastoma is the most common malignant posterior fossa tumor of childhood, classified as a WHO Grade IV embryonal tumor. Typically arising from the cerebellar vermis, this tumor fills the fourth ventricle causing obstructive hydrocephalus. Due to high cellularity, it demonstrates diffusion restriction and has a propensity for leptomeningeal dissemination (drop metastases). Molecular subgrouping (WNT, SHH, Group 3, Group 4) determines prognosis and treatment strategy. The median age at diagnosis is 5-9 years with male predominance.
Age Range
2-30
Peak Age
8
Gender
Male predominant
Prevalence
Uncommon
Medulloblastoma arises from embryonal precursor cells in the cerebellar vermis and grows toward the floor of the fourth ventricle, obstructing CSF flow. The tumor's high nuclear-to-cytoplasmic ratio and tight cell packing restrict extracellular water diffusion, producing markedly bright signal on DWI and low values on ADC maps — this feature is critical for differentiating it from other posterior fossa tumors. The dense cellularity also causes hyperdense appearance on CT and isointense-to-hypointense signal on T2-weighted MRI, as tight cell packing reduces free water content. The tumor's propensity for leptomeningeal dissemination results from cells being carried through CSF to the spinal canal and supratentorial regions; therefore, whole neuraxis imaging is mandatory. Molecular subgroups reflect different signaling pathway activations: the WNT group is characterized by β-catenin mutation and carries the best prognosis; the SHH group shows Sonic Hedgehog pathway activation and is associated with desmoplastic histology; Group 3 harbors MYC amplification and has the worst prognosis; Group 4 is the most common subgroup and is characterized by isochromosome 17q.
A posterior fossa midline mass arising from the cerebellar vermis, filling the fourth ventricle, and demonstrating marked diffusion restriction on DWI in a pediatric patient constitutes the most characteristic constellation of findings for medulloblastoma. Low ADC values (<700 × 10⁻⁶ mm²/s) reliably differentiate the tumor from ependymoma and pilocytic astrocytoma.
Hyperdense mass in the cerebellar vermis at midline on CT, filling or compressing the fourth ventricle. The tumor's dense cellularity and high nuclear-to-cytoplasmic ratio create higher density compared to brain parenchyma. Calcification is seen in 10-20% of cases, usually as small punctate calcifications. Cystic/necrotic areas are variable and more common in larger tumors. Accompanying obstructive hydrocephalus (lateral and third ventricle dilation) is almost always present and serves as an urgent diagnostic clue.
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A hyperdense mass centered in the cerebellar vermis filling the fourth ventricle is observed on CT, with accompanying obstructive hydrocephalus.
Medulloblastoma shows marked and heterogeneous enhancement on contrast-enhanced CT. Solid components enhance intensely while cystic/necrotic areas show no enhancement. This heterogeneous pattern reflects the tumor's vascular architecture and areas of necrosis. The degree of enhancement correlates with tumor grade, as neovascularization is more prominent in high-grade tumors. Areas of dural invasion or leptomeningeal dissemination are also better evaluated with contrast enhancement.
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Post-contrast, the mass demonstrates marked heterogeneous enhancement of solid components with non-enhancing internal necrotic/cystic areas.
On T1-weighted sequences, medulloblastoma shows hypointense to isointense signal relative to gray matter. The solid component appears homogeneously hypointense, while hemorrhagic areas may show T1 hyperintense signal (methemoglobin effect). Cystic/necrotic components show hypointense signal near CSF intensity. On T1 contrast-enhanced sequences, intense enhancement of solid components is observed and tumor margins are better delineated. T1 contrast-enhanced sequences are the gold standard for leptomeningeal dissemination assessment; nodular or diffuse leptomeningeal enhancement in the spinal canal indicates drop metastases.
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A midline mass centered in the cerebellar vermis shows hypointense to isointense signal on T1-weighted sequences with intense enhancement of solid components post-contrast.
On T2-weighted sequences, the solid component of medulloblastoma shows isointense to hypointense signal relative to gray matter — this feature is a distinguishing finding from other posterior fossa tumors (ependymoma and pilocytic astrocytoma are T2 hyperintense). The low T2 signal directly correlates with the tumor's dense cellularity and low free water content. Cystic/necrotic areas appear T2 hyperintense. Peritumoral edema is variable but usually prominent. The relationship of the fourth ventricle tumor with surrounding structures is best evaluated on T2.
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The solid component demonstrates isointense to hypointense signal relative to gray matter on T2-weighted sequences, suggesting high cellularity.
On diffusion-weighted imaging (DWI), medulloblastoma shows markedly bright signal with low values on ADC maps (typically 500-700 × 10⁻⁶ mm²/s) — this is the most characteristic MRI feature of medulloblastoma and is critically important for diagnosis. Diffusion restriction directly correlates with the tumor's high cellularity; tightly packed small cells restrict Brownian motion of water molecules in the extracellular space. This feature reliably differentiates medulloblastoma from ependymoma (moderate diffusion restriction) and pilocytic astrocytoma (no diffusion restriction). ADC values are inversely proportional to tumor grade and cellularity; ADC increase during treatment monitoring indicates positive response.
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The lesion demonstrates marked diffusion restriction on DWI with low signal on ADC map — consistent with a highly cellular embryonal tumor.
On FLAIR sequences, the solid component of medulloblastoma shows mild-to-moderate hyperintense signal; although free water content is reduced due to high cellularity, cellular edema and peritumoral vasogenic edema appear hyperintense. FLAIR's CSF signal suppression allows better evaluation of periventricular edema, ventricular wall involvement, and leptomeningeal dissemination. Transependymal CSF seepage (due to obstructive hydrocephalus) appears as periventricular FLAIR hyperintensity and reflects the severity of acute hydrocephalus. FLAIR also complements contrast-enhanced sequences in detecting satellite lesions and distant intracranial metastases.
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On FLAIR sequences, the mass shows mild-to-moderate hyperintense signal with peritumoral vasogenic edema and periventricular transependymal CSF seepage.
On perfusion MRI, elevated relative cerebral blood volume (rCBV) is observed in solid components of medulloblastoma. As a high-grade embryonal tumor, medulloblastoma demonstrates prominent neovascularization with rCBV ratios typically 2-4 times higher than normal brain. However, perfusion values in the posterior fossa should be interpreted cautiously due to technical challenges (bone artifact, susceptibility effects). While perfusion data aids tumor grading, DWI and conventional MRI findings are more reliable for diagnosis in posterior fossa tumors. In post-treatment follow-up, increased perfusion suggests recurrence while low perfusion suggests radiation necrosis.
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Perfusion MRI demonstrates elevated rCBV in solid components, consistent with high-grade tumor neovascularization.
On MR spectroscopy, medulloblastoma shows elevated choline (Cho) peak, reduced N-acetyl aspartate (NAA) peak, and increased Cho/NAA ratio — this pattern is consistent with high cell turnover and neuronal loss. An important finding specific to medulloblastoma is the taurine peak (3.4 ppm); taurine is elevated in embryonal tumors and has discriminatory value from other posterior fossa tumors. Lactate peak reflects necrosis and anaerobic metabolism, lipid peaks reflect cell membrane breakdown. Creatine (Cr) is generally low as rapidly metabolizing tumor cells deplete energy stores. Cho/Cr ratio > 2 strongly suggests malignancy.
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MR spectroscopy demonstrates markedly elevated choline, reduced NAA, and a taurine peak at 3.4 ppm, consistent with an embryonal tumor spectral pattern.
On SWI (Susceptibility Weighted Imaging) sequences, susceptibility foci due to calcification and/or hemorrhage may be observed in medulloblastoma. Intratumoral hemorrhage occurs in 5-10% of cases and exhibits varying signal characteristics at different stages. While calcifications are better visualized on CT, they can be detected on SWI due to diamagnetic susceptibility effects. SWI also reveals the tumor's internal vascular architecture (dilated veins, tumor vessels). This sequence is particularly helpful in differential diagnosis for excluding cavernous malformation and other hemorrhage-related lesions.
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Susceptibility foci are observed within the mass on SWI sequences, compatible with intratumoral calcification and/or hemorrhage.
Criteria
Nuclear β-catenin accumulation, monosomy 6, CTNNB1 mutation; 10% of all medulloblastomas
Distinct Features
Best prognosis subgroup (>90% 5-year survival). Cerebellopontine angle/peduncle location is typical (off-midline). Enhancement is usually homogeneous. DWI restriction is present but may be less pronounced than other subgroups. Usually seen in children aged 6-17 years.
Criteria
Sonic Hedgehog pathway activation, PTCH1/SMO/SUFU mutations; 25-30% of all medulloblastomas
Distinct Features
Bimodal age distribution: infantile (<3 years) and adult (>17 years). Cerebellar hemisphere location (lateral) is characteristic — desmoplastic/nodular histology is common. Nodular enhancement pattern and cystic components may be prominent on MRI. In infantile form, desmoplastic histology carries good prognosis, while TP53 mutation in adult form is associated with poor prognosis. Hedgehog inhibitors (vismodegib) may be used in treatment.
Criteria
MYC amplification, isochromosome 17q; 25% of all medulloblastomas; worst prognosis subgroup
Distinct Features
Worst prognosis subgroup (50-60% 5-year survival). Classic midline vermian location is typical. Large tumors with cystic components are common. High rate of leptomeningeal metastasis at diagnosis (40-50%). Demonstrates marked DWI restriction and heterogeneous enhancement on MRI. Male predominance is pronounced. Typically seen in children aged 3-10 years and rare in infants or adults.
Criteria
Isochromosome 17q, MYCN amplification (rare); 35-40% of all medulloblastomas; most common subgroup
Distinct Features
Most common subgroup but with the least understood molecular mechanism. Midline vermian location is typical. Enhancement is variable and may show minimal enhancement (unlike other subgroups). Intermediate prognosis (70-80% 5-year survival). Metastasis rate at diagnosis is lower than Group 3 (30-35%). Male predominance is pronounced (3:1). Typically seen in children aged 5-15 years.
Distinguishing Feature
Ependymoma arises from the floor of the fourth ventricle and extends plastically through the foramina of Luschka; DWI restriction is less pronounced than medulloblastoma (higher ADC values); calcification and cystic changes are more frequent; shows heterogeneous enhancement
Distinguishing Feature
Pilocytic astrocytoma usually located in the cerebellar hemisphere (not vermis), cyst + mural nodule pattern is typical, does not show diffusion restriction on DWI (high ADC), mural nodule shows intense homogeneous enhancement; better prognosis (WHO Grade I)
Distinguishing Feature
Hemangioblastoma usually occurs in adults (except VHL syndrome), cyst + intensely enhancing mural nodule pattern is typical, no diffusion restriction on DWI, prominent flow voids present (vascular tumor), does not demonstrate leptomeningeal dissemination
Distinguishing Feature
Primary CNS lymphoma is more common in adults, periventricular/deep gray matter location is typical (posterior fossa less common), shows DWI restriction but intense homogeneous enhancement is characteristic, usually presents as solitary or multifocal supratentorial mass; dramatic response to steroid therapy ('ghost tumor')
Distinguishing Feature
Metastases are much more common in adults, usually multiple and located at gray-white matter junction; surrounding edema is disproportionately large relative to mass, ring enhancement is common; known primary malignancy history is critical for diagnosis; posterior fossa metastasis is rare in children
Urgency
emergencyManagement
surgical-oncologicalBiopsy
Not NeededFollow-up
Cerrahi rezeksiyon sonrası kraniospinal radyoterapi ve kemoterapi; 3 ayda bir MR izlem (ilk 2 yıl), sonra 6 ayda bir; nöraksın tamamı değerlendirilmeliMedulloblastoma is an emergency requiring urgent intervention; obstructive hydrocephalus can be life-threatening. Standard treatment is maximal safe surgical resection followed by craniospinal radiotherapy and chemotherapy based on age and risk group. Molecular subgrouping (WNT/SHH/Group 3/Group 4) determines treatment intensity and prognostic assessment. While the WNT group is being studied for de-escalation, Group 3 requires intensified treatment. Preoperative whole neuraxis MRI and CSF cytology are mandatory for staging. Recurrence and leptomeningeal dissemination must be detected early during follow-up.
Medulloblastoma is treated with surgical resection, craniospinal radiation, and chemotherapy. Five-year survival is 60-80% (varies by risk group). Molecular subtypes (WNT, SHH, Group 3, Group 4) determine prognosis and treatment strategy. WNT subtype has the best prognosis.